diff --git a/src/builtins/builtins-array.cc b/src/builtins/builtins-array.cc
index dcc691e2ef88..ceeee5f37db8 100644
--- a/src/builtins/builtins-array.cc
+++ b/src/builtins/builtins-array.cc
@@ -316,7 +316,7 @@ V8_WARN_UNUSED_RESULT Object* GenericArrayPush(Isolate* isolate,
   Handle<Object> raw_length_number;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
       isolate, raw_length_number,
-      Object::GetLengthFromArrayLike(isolate, Handle<Object>::cast(receiver)));
+      Object::GetLengthFromArrayLike(isolate, receiver));
 
   // 3. Let args be a List whose elements are, in left to right order,
   //    the arguments that were passed to this function invocation.
diff --git a/src/builtins/builtins-intl.cc b/src/builtins/builtins-intl.cc
index 9362f73143e7..d59e4978e77c 100644
--- a/src/builtins/builtins-intl.cc
+++ b/src/builtins/builtins-intl.cc
@@ -41,7 +41,7 @@ BUILTIN(StringPrototypeToUpperCaseIntl) {
   HandleScope scope(isolate);
   TO_THIS_STRING(string, "String.prototype.toUpperCase");
   string = String::Flatten(isolate, string);
-  return ConvertCase(string, true, isolate);
+  RETURN_RESULT_OR_FAILURE(isolate, ConvertCase(string, true, isolate));
 }
 
 BUILTIN(StringPrototypeNormalizeIntl) {
diff --git a/src/contexts.h b/src/contexts.h
index 65a427f54563..c0a730e590a1 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -90,7 +90,6 @@ enum ContextLookupFlags {
   V(MAP_HAS_INDEX, JSFunction, map_has)                                        \
   V(MAP_SET_INDEX, JSFunction, map_set)                                        \
   V(FUNCTION_HAS_INSTANCE_INDEX, JSFunction, function_has_instance)            \
-  V(INITIALIZE_LOCALE_LIST_FUNCTION_INDEX, JSFunction, initialize_locale_list) \
   V(OBJECT_VALUE_OF, JSFunction, object_value_of)                              \
   V(OBJECT_TO_STRING, JSFunction, object_to_string)                            \
   V(PROMISE_CATCH_INDEX, JSFunction, promise_catch)                            \
diff --git a/src/intl.cc b/src/intl.cc
index 985752577211..c8548b6d4805 100644
--- a/src/intl.cc
+++ b/src/intl.cc
@@ -155,10 +155,8 @@ const UChar* GetUCharBufferFromFlat(const String::FlatContent& flat,
   }
 }
 
-V8_WARN_UNUSED_RESULT Object* LocaleConvertCase(Handle<String> s,
-                                                Isolate* isolate,
-                                                bool is_to_upper,
-                                                const char* lang) {
+MaybeHandle<String> LocaleConvertCase(Handle<String> s, Isolate* isolate,
+                                      bool is_to_upper, const char* lang) {
   auto case_converter = is_to_upper ? u_strToUpper : u_strToLower;
   int32_t src_length = s->length();
   int32_t dest_length = src_length;
@@ -166,15 +164,16 @@ V8_WARN_UNUSED_RESULT Object* LocaleConvertCase(Handle<String> s,
   Handle<SeqTwoByteString> result;
   std::unique_ptr<uc16[]> sap;
 
-  if (dest_length == 0) return ReadOnlyRoots(isolate).empty_string();
+  if (dest_length == 0) return ReadOnlyRoots(isolate).empty_string_handle();
 
   // This is not a real loop. It'll be executed only once (no overflow) or
   // twice (overflow).
   for (int i = 0; i < 2; ++i) {
     // Case conversion can increase the string length (e.g. sharp-S => SS) so
     // that we have to handle RangeError exceptions here.
-    ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
-        isolate, result, isolate->factory()->NewRawTwoByteString(dest_length));
+    ASSIGN_RETURN_ON_EXCEPTION(
+        isolate, result, isolate->factory()->NewRawTwoByteString(dest_length),
+        String);
     DisallowHeapAllocation no_gc;
     DCHECK(s->IsFlat());
     String::FlatContent flat = s->GetFlatContent();
@@ -192,21 +191,17 @@ V8_WARN_UNUSED_RESULT Object* LocaleConvertCase(Handle<String> s,
   DCHECK(U_SUCCESS(status));
   if (V8_LIKELY(status == U_STRING_NOT_TERMINATED_WARNING)) {
     DCHECK(dest_length == result->length());
-    return *result;
+    return result;
   }
-  if (U_SUCCESS(status)) {
-    DCHECK(dest_length < result->length());
-    return *Handle<SeqTwoByteString>::cast(
-        SeqString::Truncate(result, dest_length));
-  }
-  return *s;
+  DCHECK(dest_length < result->length());
+  return SeqString::Truncate(result, dest_length);
 }
 
 // A stripped-down version of ConvertToLower that can only handle flat one-byte
 // strings and does not allocate. Note that {src} could still be, e.g., a
 // one-byte sliced string with a two-byte parent string.
 // Called from TF builtins.
-V8_WARN_UNUSED_RESULT Object* ConvertOneByteToLower(String* src, String* dst) {
+V8_WARN_UNUSED_RESULT String* ConvertOneByteToLower(String* src, String* dst) {
   DCHECK_EQ(src->length(), dst->length());
   DCHECK(src->HasOnlyOneByteChars());
   DCHECK(src->IsFlat());
@@ -251,8 +246,7 @@ V8_WARN_UNUSED_RESULT Object* ConvertOneByteToLower(String* src, String* dst) {
   return dst;
 }
 
-V8_WARN_UNUSED_RESULT Object* ConvertToLower(Handle<String> s,
-                                             Isolate* isolate) {
+MaybeHandle<String> ConvertToLower(Handle<String> s, Isolate* isolate) {
   if (!s->HasOnlyOneByteChars()) {
     // Use a slower implementation for strings with characters beyond U+00FF.
     return LocaleConvertCase(s, isolate, false, "");
@@ -274,17 +268,16 @@ V8_WARN_UNUSED_RESULT Object* ConvertToLower(Handle<String> s,
   bool is_short = length < static_cast<int>(sizeof(uintptr_t));
   if (is_short) {
     bool is_lower_ascii = FindFirstUpperOrNonAscii(*s, length) == length;
-    if (is_lower_ascii) return *s;
+    if (is_lower_ascii) return s;
   }
 
   Handle<SeqOneByteString> result =
       isolate->factory()->NewRawOneByteString(length).ToHandleChecked();
 
-  return ConvertOneByteToLower(*s, *result);
+  return Handle<String>(ConvertOneByteToLower(*s, *result), isolate);
 }
 
-V8_WARN_UNUSED_RESULT Object* ConvertToUpper(Handle<String> s,
-                                             Isolate* isolate) {
+MaybeHandle<String> ConvertToUpper(Handle<String> s, Isolate* isolate) {
   int32_t length = s->length();
   if (s->HasOnlyOneByteChars() && length > 0) {
     Handle<SeqOneByteString> result =
@@ -304,8 +297,9 @@ V8_WARN_UNUSED_RESULT Object* ConvertToUpper(Handle<String> s,
             FastAsciiConvert<false>(reinterpret_cast<char*>(result->GetChars()),
                                     reinterpret_cast<const char*>(src.start()),
                                     length, &has_changed_character);
-        if (index_to_first_unprocessed == length)
-          return has_changed_character ? *result : *s;
+        if (index_to_first_unprocessed == length) {
+          return has_changed_character ? result : s;
+        }
         // If not ASCII, we keep the result up to index_to_first_unprocessed and
         // process the rest.
         is_result_single_byte =
@@ -314,7 +308,7 @@ V8_WARN_UNUSED_RESULT Object* ConvertToUpper(Handle<String> s,
       } else {
         DCHECK(flat.IsTwoByte());
         Vector<const uint16_t> src = flat.ToUC16Vector();
-        if (ToUpperFastASCII(src, result)) return *result;
+        if (ToUpperFastASCII(src, result)) return result;
         is_result_single_byte = ToUpperOneByte(src, dest, &sharp_s_count);
       }
     }
@@ -325,13 +319,14 @@ V8_WARN_UNUSED_RESULT Object* ConvertToUpper(Handle<String> s,
       return LocaleConvertCase(s, isolate, true, "");
     }
 
-    if (sharp_s_count == 0) return *result;
+    if (sharp_s_count == 0) return result;
 
     // We have sharp_s_count sharp-s characters, but the result is still
     // in the Latin-1 range.
-    ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
+    ASSIGN_RETURN_ON_EXCEPTION(
         isolate, result,
-        isolate->factory()->NewRawOneByteString(length + sharp_s_count));
+        isolate->factory()->NewRawOneByteString(length + sharp_s_count),
+        String);
     DisallowHeapAllocation no_gc;
     String::FlatContent flat = s->GetFlatContent();
     if (flat.IsOneByte()) {
@@ -340,14 +335,14 @@ V8_WARN_UNUSED_RESULT Object* ConvertToUpper(Handle<String> s,
       ToUpperWithSharpS(flat.ToUC16Vector(), result);
     }
 
-    return *result;
+    return result;
   }
 
   return LocaleConvertCase(s, isolate, true, "");
 }
 
-V8_WARN_UNUSED_RESULT Object* ConvertCase(Handle<String> s, bool is_upper,
-                                          Isolate* isolate) {
+MaybeHandle<String> ConvertCase(Handle<String> s, bool is_upper,
+                                Isolate* isolate) {
   return is_upper ? ConvertToUpper(s, isolate) : ConvertToLower(s, isolate);
 }
 
diff --git a/src/intl.h b/src/intl.h
index ca6478d247d1..5ec5381f409a 100644
--- a/src/intl.h
+++ b/src/intl.h
@@ -37,18 +37,14 @@ enum class IcuService {
 const UChar* GetUCharBufferFromFlat(const String::FlatContent& flat,
                                     std::unique_ptr<uc16[]>* dest,
                                     int32_t length);
-V8_WARN_UNUSED_RESULT Object* LocaleConvertCase(Handle<String> s,
-                                                Isolate* isolate,
-                                                bool is_to_upper,
-                                                const char* lang);
-V8_WARN_UNUSED_RESULT Object* ConvertToLower(Handle<String> s,
-                                             Isolate* isolate);
-V8_WARN_UNUSED_RESULT Object* ConvertToUpper(Handle<String> s,
-                                             Isolate* isolate);
-V8_WARN_UNUSED_RESULT Object* ConvertCase(Handle<String> s, bool is_upper,
-                                          Isolate* isolate);
-
-V8_WARN_UNUSED_RESULT Object* ConvertOneByteToLower(String* src, String* dst);
+MaybeHandle<String> LocaleConvertCase(Handle<String> s, Isolate* isolate,
+                                      bool is_to_upper, const char* lang);
+MaybeHandle<String> ConvertToLower(Handle<String> s, Isolate* isolate);
+MaybeHandle<String> ConvertToUpper(Handle<String> s, Isolate* isolate);
+MaybeHandle<String> ConvertCase(Handle<String> s, bool is_upper,
+                                Isolate* isolate);
+
+V8_WARN_UNUSED_RESULT String* ConvertOneByteToLower(String* src, String* dst);
 
 const uint8_t* ToLatin1LowerTable();
 
diff --git a/src/js/intl.js b/src/js/intl.js
index d9b09fff7e65..77a52ec80e37 100644
--- a/src/js/intl.js
+++ b/src/js/intl.js
@@ -772,13 +772,6 @@ function initializeLocaleList(locales) {
   return freezeArray(canonicalizeLocaleList(locales));
 }
 
-// TODO(ftang): remove the %InstallToContext once
-// initializeLocaleList is available in C++
-// https://bugs.chromium.org/p/v8/issues/detail?id=7987
-%InstallToContext([
-  "initialize_locale_list", initializeLocaleList
-]);
-
 // ECMA 402 section 8.2.1
 DEFINE_METHOD(
   GlobalIntl,
diff --git a/src/json-stringifier.cc b/src/json-stringifier.cc
index 0a84be99a50e..91a1b7201bfe 100644
--- a/src/json-stringifier.cc
+++ b/src/json-stringifier.cc
@@ -226,7 +226,9 @@ bool JsonStringifier::InitializeReplacer(Handle<Object> replacer) {
     Handle<Object> length_obj;
     ASSIGN_RETURN_ON_EXCEPTION_VALUE(
         isolate_, length_obj,
-        Object::GetLengthFromArrayLike(isolate_, replacer), false);
+        Object::GetLengthFromArrayLike(isolate_,
+                                       Handle<JSReceiver>::cast(replacer)),
+        false);
     uint32_t length;
     if (!length_obj->ToUint32(&length)) length = kMaxUInt32;
     for (uint32_t i = 0; i < length; i++) {
@@ -720,7 +722,9 @@ JsonStringifier::Result JsonStringifier::SerializeJSProxy(
     Handle<Object> length_object;
     ASSIGN_RETURN_ON_EXCEPTION_VALUE(
         isolate_, length_object,
-        Object::GetLengthFromArrayLike(isolate_, object), EXCEPTION);
+        Object::GetLengthFromArrayLike(isolate_,
+                                       Handle<JSReceiver>::cast(object)),
+        EXCEPTION);
     uint32_t length;
     if (!length_object->ToUint32(&length)) {
       // Technically, we need to be able to handle lengths outside the
diff --git a/src/objects.cc b/src/objects.cc
index 587d176598ba..88fc927b95ce 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -953,11 +953,11 @@ MaybeHandle<FixedArray> Object::CreateListFromArrayLike(
 
 // static
 MaybeHandle<Object> Object::GetLengthFromArrayLike(Isolate* isolate,
-                                                   Handle<Object> object) {
+                                                   Handle<JSReceiver> object) {
   Handle<Object> val;
-  Handle<Object> key = isolate->factory()->length_string();
+  Handle<Name> key = isolate->factory()->length_string();
   ASSIGN_RETURN_ON_EXCEPTION(
-      isolate, val, Runtime::GetObjectProperty(isolate, object, key), Object);
+      isolate, val, JSReceiver::GetProperty(isolate, object, key), Object);
   return Object::ToLength(isolate, val);
 }
 
diff --git a/src/objects.h b/src/objects.h
index a9933df98563..bf27eee12dc4 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1289,7 +1289,7 @@ class Object {
 
   // Get length property and apply ToLength.
   V8_WARN_UNUSED_RESULT static MaybeHandle<Object> GetLengthFromArrayLike(
-      Isolate* isolate, Handle<Object> object);
+      Isolate* isolate, Handle<JSReceiver> object);
 
   // ES6 section 12.5.6 The typeof Operator
   static Handle<String> TypeOf(Isolate* isolate, Handle<Object> object);
diff --git a/src/objects/intl-objects.cc b/src/objects/intl-objects.cc
index a443788a5f9a..3bd80532be6b 100644
--- a/src/objects/intl-objects.cc
+++ b/src/objects/intl-objects.cc
@@ -9,7 +9,9 @@
 #include "src/objects/intl-objects.h"
 #include "src/objects/intl-objects-inl.h"
 
+#include <algorithm>
 #include <memory>
+#include <string>
 
 #include "src/api-inl.h"
 #include "src/global-handles.h"
@@ -1073,7 +1075,7 @@ V8_WARN_UNUSED_RESULT MaybeHandle<JSObject> Intl::AvailableLocalesOf(
   return locales;
 }
 
-V8_WARN_UNUSED_RESULT Handle<String> Intl::DefaultLocale(Isolate* isolate) {
+std::string Intl::DefaultLocale(Isolate* isolate) {
   if (isolate->default_locale().empty()) {
     icu::Locale default_locale;
     // Translate ICU's fallback locale to a well-known locale.
@@ -1091,8 +1093,7 @@ V8_WARN_UNUSED_RESULT Handle<String> Intl::DefaultLocale(Isolate* isolate) {
     }
     DCHECK(!isolate->default_locale().empty());
   }
-  return isolate->factory()->NewStringFromAsciiChecked(
-      isolate->default_locale().c_str());
+  return isolate->default_locale();
 }
 
 bool Intl::IsObjectOfType(Isolate* isolate, Handle<Object> input,
@@ -1354,8 +1355,8 @@ MaybeHandle<JSObject> Intl::ResolveLocale(Isolate* isolate, const char* service,
   return Handle<JSObject>::cast(result);
 }
 
-MaybeHandle<JSObject> Intl::CanonicalizeLocaleList(Isolate* isolate,
-                                                   Handle<Object> locales) {
+MaybeHandle<JSObject> Intl::CanonicalizeLocaleListJS(Isolate* isolate,
+                                                     Handle<Object> locales) {
   Handle<JSFunction> canonicalize_locale_list_function =
       isolate->canonicalize_locale_list();
 
@@ -1586,17 +1587,26 @@ bool IsGrandfatheredTagWithoutPreferredVaule(const std::string& locale) {
 
 }  // anonymous namespace
 
-MaybeHandle<String> Intl::CanonicalizeLanguageTag(Isolate* isolate,
-                                                  Handle<Object> locale_in) {
+Maybe<std::string> Intl::CanonicalizeLanguageTag(Isolate* isolate,
+                                                 Handle<Object> locale_in) {
   Handle<String> locale_str;
+  // This does part of the validity checking spec'ed in CanonicalizeLocaleList:
+  // 7c ii. If Type(kValue) is not String or Object, throw a TypeError
+  // exception.
+  // 7c iii. Let tag be ? ToString(kValue).
+  // 7c iv. If IsStructurallyValidLanguageTag(tag) is false, throw a
+  // RangeError exception.
+
   if (locale_in->IsString()) {
     locale_str = Handle<String>::cast(locale_in);
   } else if (locale_in->IsJSReceiver()) {
-    ASSIGN_RETURN_ON_EXCEPTION(isolate, locale_str,
-                               Object::ToString(isolate, locale_in), String);
+    ASSIGN_RETURN_ON_EXCEPTION_VALUE(isolate, locale_str,
+                                     Object::ToString(isolate, locale_in),
+                                     Nothing<std::string>());
   } else {
-    THROW_NEW_ERROR(isolate, NewTypeError(MessageTemplate::kLanguageID),
-                    String);
+    THROW_NEW_ERROR_RETURN_VALUE(isolate,
+                                 NewTypeError(MessageTemplate::kLanguageID),
+                                 Nothing<std::string>());
   }
   std::string locale(locale_str->ToCString().get());
 
@@ -1607,24 +1617,25 @@ MaybeHandle<String> Intl::CanonicalizeLanguageTag(Isolate* isolate,
   // fast-track 'fil' (3-letter canonical code).
   if ((IsTwoLetterLanguage(locale) && !IsDeprecatedLanguage(locale)) ||
       locale == "fil") {
-    return locale_str;
+    return Just(locale);
   }
 
   // Because per BCP 47 2.1.1 language tags are case-insensitive, lowercase
   // the input before any more check.
   std::transform(locale.begin(), locale.end(), locale.begin(), AsciiToLower);
   if (!IsStructurallyValidLanguageTag(isolate, locale)) {
-    THROW_NEW_ERROR(
+    THROW_NEW_ERROR_RETURN_VALUE(
         isolate,
         NewRangeError(MessageTemplate::kInvalidLanguageTag, locale_str),
-        String);
+        Nothing<std::string>());
   }
 
   // ICU maps a few grandfathered tags to what looks like a regular language
   // tag even though IANA language tag registry does not have a preferred
   // entry map for them. Return them as they're with lowercasing.
-  if (IsGrandfatheredTagWithoutPreferredVaule(locale))
-    return isolate->factory()->NewStringFromAsciiChecked(locale.data());
+  if (IsGrandfatheredTagWithoutPreferredVaule(locale)) {
+    return Just(locale);
+  }
 
   // // ECMA 402 6.2.3
   // TODO(jshin): uloc_{for,to}TanguageTag can fail even for a structually valid
@@ -1640,10 +1651,10 @@ MaybeHandle<String> Intl::CanonicalizeLanguageTag(Isolate* isolate,
   if (U_FAILURE(error) || error == U_STRING_NOT_TERMINATED_WARNING) {
     // TODO(jshin): This should not happen because the structural validity
     // is already checked. If that's the case, remove this.
-    THROW_NEW_ERROR(
+    THROW_NEW_ERROR_RETURN_VALUE(
         isolate,
         NewRangeError(MessageTemplate::kInvalidLanguageTag, locale_str),
-        String);
+        Nothing<std::string>());
   }
 
   // Force strict BCP47 rules.
@@ -1652,15 +1663,87 @@ MaybeHandle<String> Intl::CanonicalizeLanguageTag(Isolate* isolate,
                                           ULOC_FULLNAME_CAPACITY, TRUE, &error);
 
   if (U_FAILURE(error)) {
-    THROW_NEW_ERROR(
+    THROW_NEW_ERROR_RETURN_VALUE(
         isolate,
         NewRangeError(MessageTemplate::kInvalidLanguageTag, locale_str),
-        String);
+        Nothing<std::string>());
   }
 
-  return isolate->factory()
-      ->NewStringFromOneByte(OneByteVector(result, result_len), NOT_TENURED)
-      .ToHandleChecked();
+  return Just(std::string(result, result_len));
+}
+
+Maybe<std::vector<std::string>> Intl::CanonicalizeLocaleList(
+    Isolate* isolate, Handle<Object> locales, bool only_return_one_result) {
+  // 1. If locales is undefined, then
+  if (locales->IsUndefined(isolate)) {
+    // 1a. Return a new empty List.
+    return Just(std::vector<std::string>());
+  }
+  // 2. Let seen be a new empty List.
+  std::vector<std::string> seen;
+  // 3. If Type(locales) is String, then
+  if (locales->IsString()) {
+    // 3a. Let O be CreateArrayFromList(« locales »).
+    // Instead of creating a one-element array and then iterating over it,
+    // we inline the body of the iteration:
+    std::string canonicalized_tag;
+    if (!CanonicalizeLanguageTag(isolate, locales).To(&canonicalized_tag)) {
+      return Nothing<std::vector<std::string>>();
+    }
+    seen.push_back(canonicalized_tag);
+    return Just(seen);
+  }
+  // 4. Else,
+  // 4a. Let O be ? ToObject(locales).
+  Handle<JSReceiver> o;
+  ASSIGN_RETURN_ON_EXCEPTION_VALUE(isolate, o,
+                                   Object::ToObject(isolate, locales),
+                                   Nothing<std::vector<std::string>>());
+  // 5. Let len be ? ToLength(? Get(O, "length")).
+  Handle<Object> length_obj;
+  ASSIGN_RETURN_ON_EXCEPTION_VALUE(isolate, length_obj,
+                                   Object::GetLengthFromArrayLike(isolate, o),
+                                   Nothing<std::vector<std::string>>());
+  // TODO(jkummerow): Spec violation: strictly speaking, we have to iterate
+  // up to 2^53-1 if {length_obj} says so. Since cases above 2^32 probably
+  // don't happen in practice (and would be very slow if they do), we'll keep
+  // the code simple for now by using a saturating to-uint32 conversion.
+  double raw_length = length_obj->Number();
+  uint32_t len =
+      raw_length >= kMaxUInt32 ? kMaxUInt32 : static_cast<uint32_t>(raw_length);
+  // 6. Let k be 0.
+  // 7. Repeat, while k < len
+  for (uint32_t k = 0; k < len; k++) {
+    // 7a. Let Pk be ToString(k).
+    // 7b. Let kPresent be ? HasProperty(O, Pk).
+    LookupIterator it(isolate, o, k);
+    // 7c. If kPresent is true, then
+    if (!it.IsFound()) continue;
+    // 7c i. Let kValue be ? Get(O, Pk).
+    Handle<Object> k_value;
+    ASSIGN_RETURN_ON_EXCEPTION_VALUE(isolate, k_value, Object::GetProperty(&it),
+                                     Nothing<std::vector<std::string>>());
+    // 7c ii. If Type(kValue) is not String or Object, throw a TypeError
+    // exception.
+    // 7c iii. Let tag be ? ToString(kValue).
+    // 7c iv. If IsStructurallyValidLanguageTag(tag) is false, throw a
+    // RangeError exception.
+    // 7c v. Let canonicalizedTag be CanonicalizeLanguageTag(tag).
+    std::string canonicalized_tag;
+    if (!CanonicalizeLanguageTag(isolate, k_value).To(&canonicalized_tag)) {
+      return Nothing<std::vector<std::string>>();
+    }
+    // 7c vi. If canonicalizedTag is not an element of seen, append
+    // canonicalizedTag as the last element of seen.
+    if (std::find(seen.begin(), seen.end(), canonicalized_tag) == seen.end()) {
+      seen.push_back(canonicalized_tag);
+    }
+    // 7d. Increase k by 1. (See loop header.)
+    // Optimization: some callers only need one result.
+    if (only_return_one_result) return Just(seen);
+  }
+  // 8. Return seen.
+  return Just(seen);
 }
 
 // ecma-402/#sec-currencydigits
@@ -1707,32 +1790,16 @@ MaybeHandle<JSObject> Intl::CreateNumberFormat(Isolate* isolate,
 
 namespace {
 
-// Remove the following after we port InitializeLocaleList from src/js/intl.js
-// to c++ https://bugs.chromium.org/p/v8/issues/detail?id=7987
+bool IsAToZ(char ch) {
+  return IsInRange(AsciiAlphaToLower(ch), 'a', 'z');
+}
+
 // The following are temporary function calling back into js code in
 // src/js/intl.js to call pre-existing functions until they are all moved to C++
 // under src/objects/*.
 // TODO(ftang): remove these temp function after bstell move them from js into
 // C++
 
-MaybeHandle<JSObject> InitializeLocaleList(Isolate* isolate,
-                                           Handle<Object> list) {
-  Handle<Object> result;
-  Handle<Object> undefined_value(ReadOnlyRoots(isolate).undefined_value(),
-                                 isolate);
-  Handle<Object> args[] = {list};
-  ASSIGN_RETURN_ON_EXCEPTION(
-      isolate, result,
-      Execution::Call(isolate, isolate->initialize_locale_list(),
-                      undefined_value, arraysize(args), args),
-      JSArray);
-  return Handle<JSObject>::cast(result);
-}
-
-bool IsAToZ(char ch) {
-  return (('A' <= ch) && (ch <= 'Z')) || (('a' <= ch) && (ch <= 'z'));
-}
-
 MaybeHandle<JSObject> CachedOrNewService(Isolate* isolate,
                                          Handle<String> service,
                                          Handle<Object> locales,
@@ -1777,85 +1844,40 @@ MaybeHandle<String> Intl::StringLocaleConvertCase(Isolate* isolate,
                                                   Handle<String> s,
                                                   bool to_upper,
                                                   Handle<Object> locales) {
-  Factory* factory = isolate->factory();
-  Handle<String> requested_locale;
-  if (locales->IsUndefined()) {
-    requested_locale = Intl::DefaultLocale(isolate);
-  } else if (locales->IsString()) {
-    ASSIGN_RETURN_ON_EXCEPTION(isolate, requested_locale,
-                               CanonicalizeLanguageTag(isolate, locales),
-                               String);
-  } else {
-    Handle<JSObject> list;
-    ASSIGN_RETURN_ON_EXCEPTION(isolate, list,
-                               InitializeLocaleList(isolate, locales), String);
-    Handle<Object> length;
-    ASSIGN_RETURN_ON_EXCEPTION(
-        isolate, length, Object::GetLengthFromArrayLike(isolate, list), String);
-    if (length->Number() > 0) {
-      Handle<Object> element;
-      ASSIGN_RETURN_ON_EXCEPTION(
-          isolate, element,
-          JSObject::GetPropertyOrElement(
-              isolate, list, factory->NumberToString(factory->NewNumber(0))),
-          String);
-      ASSIGN_RETURN_ON_EXCEPTION(isolate, requested_locale,
-                                 Object::ToString(isolate, element), String);
-    } else {
-      requested_locale = Intl::DefaultLocale(isolate);
-    }
+  std::vector<std::string> requested_locales;
+  if (!CanonicalizeLocaleList(isolate, locales, true).To(&requested_locales)) {
+    return MaybeHandle<String>();
   }
-  int dash = String::IndexOf(isolate, requested_locale,
-                             factory->NewStringFromStaticChars("-"), 0);
-  if (dash > 0) {
-    requested_locale = factory->NewSubString(requested_locale, 0, dash);
+  std::string requested_locale = requested_locales.size() == 0
+                                     ? Intl::DefaultLocale(isolate)
+                                     : requested_locales[0];
+  size_t dash = requested_locale.find("-");
+  if (dash != std::string::npos) {
+    requested_locale = requested_locale.substr(0, dash);
   }
 
   // Primary language tag can be up to 8 characters long in theory.
   // https://tools.ietf.org/html/bcp47#section-2.2.1
-  DCHECK_LE(requested_locale->length(), 8);
-  requested_locale = String::Flatten(isolate, requested_locale);
+  DCHECK_LE(requested_locale.length(), 8);
   s = String::Flatten(isolate, s);
 
   // All the languages requiring special-handling have two-letter codes.
   // Note that we have to check for '!= 2' here because private-use language
   // tags (x-foo) or grandfathered irregular tags (e.g. i-enochian) would have
   // only 'x' or 'i' when they get here.
-  if (V8_UNLIKELY(requested_locale->length() != 2)) {
-    Handle<Object> obj(ConvertCase(s, to_upper, isolate), isolate);
-    return Object::ToString(isolate, obj);
-  }
-
-  char c1, c2;
-  {
-    DisallowHeapAllocation no_gc;
-    String::FlatContent lang = requested_locale->GetFlatContent();
-    c1 = lang.Get(0);
-    c2 = lang.Get(1);
+  if (V8_UNLIKELY(requested_locale.length() != 2)) {
+    return ConvertCase(s, to_upper, isolate);
   }
   // TODO(jshin): Consider adding a fast path for ASCII or Latin-1. The fastpath
   // in the root locale needs to be adjusted for az, lt and tr because even case
   // mapping of ASCII range characters are different in those locales.
   // Greek (el) does not require any adjustment.
-  if (V8_UNLIKELY(c1 == 't' && c2 == 'r')) {
-    Handle<Object> obj(LocaleConvertCase(s, isolate, to_upper, "tr"), isolate);
-    return Object::ToString(isolate, obj);
-  }
-  if (V8_UNLIKELY(c1 == 'e' && c2 == 'l')) {
-    Handle<Object> obj(LocaleConvertCase(s, isolate, to_upper, "el"), isolate);
-    return Object::ToString(isolate, obj);
-  }
-  if (V8_UNLIKELY(c1 == 'l' && c2 == 't')) {
-    Handle<Object> obj(LocaleConvertCase(s, isolate, to_upper, "lt"), isolate);
-    return Object::ToString(isolate, obj);
-  }
-  if (V8_UNLIKELY(c1 == 'a' && c2 == 'z')) {
-    Handle<Object> obj(LocaleConvertCase(s, isolate, to_upper, "az"), isolate);
-    return Object::ToString(isolate, obj);
+  if (V8_UNLIKELY((requested_locale == "tr") || (requested_locale == "el") ||
+                  (requested_locale == "lt") || (requested_locale == "az"))) {
+    return LocaleConvertCase(s, isolate, to_upper, requested_locale.c_str());
+  } else {
+    return ConvertCase(s, to_upper, isolate);
   }
-
-  Handle<Object> obj(ConvertCase(s, to_upper, isolate), isolate);
-  return Object::ToString(isolate, obj);
 }
 
 MaybeHandle<Object> Intl::StringLocaleCompare(Isolate* isolate,
diff --git a/src/objects/intl-objects.h b/src/objects/intl-objects.h
index 77c963e30b45..3666d604aacb 100644
--- a/src/objects/intl-objects.h
+++ b/src/objects/intl-objects.h
@@ -195,7 +195,7 @@ class Intl {
   static V8_WARN_UNUSED_RESULT MaybeHandle<JSObject> AvailableLocalesOf(
       Isolate* isolate, Handle<String> service);
 
-  static V8_WARN_UNUSED_RESULT Handle<String> DefaultLocale(Isolate* isolate);
+  static std::string DefaultLocale(Isolate* isolate);
 
   static void DefineWEProperty(Isolate* isolate, Handle<JSObject> target,
                                Handle<Name> key, Handle<Object> value);
@@ -234,9 +234,12 @@ class Intl {
 
   // This currently calls out to the JavaScript implementation of
   // CanonicalizeLocaleList.
+  // Note: This is deprecated glue code, required only as long as ResolveLocale
+  // still calls a JS implementation. The C++ successor is the overloaded
+  // version below that returns a Maybe<std::vector<std::string>>.
   //
   // ecma402/#sec-canonicalizelocalelist
-  V8_WARN_UNUSED_RESULT static MaybeHandle<JSObject> CanonicalizeLocaleList(
+  V8_WARN_UNUSED_RESULT static MaybeHandle<JSObject> CanonicalizeLocaleListJS(
       Isolate* isolate, Handle<Object> locales);
 
   // ECMA402 9.2.10. GetOption( options, property, type, values, fallback)
@@ -275,9 +278,18 @@ class Intl {
       Isolate* isolate, Handle<JSReceiver> options, const char* property,
       const char* service, bool* result);
 
-  // Canonicalize the localeID.
-  static MaybeHandle<String> CanonicalizeLanguageTag(Isolate* isolate,
-                                                     Handle<Object> localeID);
+  // Canonicalize the locale.
+  // https://tc39.github.io/ecma402/#sec-canonicalizelanguagetag,
+  // including type check and structural validity check.
+  static Maybe<std::string> CanonicalizeLanguageTag(Isolate* isolate,
+                                                    Handle<Object> locale_in);
+
+  // https://tc39.github.io/ecma402/#sec-canonicalizelocalelist
+  // {only_return_one_result} is an optimization for callers that only
+  // care about the first result.
+  static Maybe<std::vector<std::string>> CanonicalizeLocaleList(
+      Isolate* isolate, Handle<Object> locales,
+      bool only_return_one_result = false);
 
   // ecma-402/#sec-currencydigits
   // The currency is expected to an all upper case string value.
diff --git a/src/objects/js-plural-rules.cc b/src/objects/js-plural-rules.cc
index 852f3136ef1f..07cc62a41e60 100644
--- a/src/objects/js-plural-rules.cc
+++ b/src/objects/js-plural-rules.cc
@@ -89,9 +89,11 @@ MaybeHandle<JSPluralRules> JSPluralRules::InitializePluralRules(
     Isolate* isolate, Handle<JSPluralRules> plural_rules,
     Handle<Object> locales, Handle<Object> options_obj) {
   // 1. Let requestedLocales be ? CanonicalizeLocaleList(locales).
+  // TODO(jkummerow): Port ResolveLocale, then use the C++ version of
+  // CanonicalizeLocaleList here.
   Handle<JSObject> requested_locales;
   ASSIGN_RETURN_ON_EXCEPTION(isolate, requested_locales,
-                             Intl::CanonicalizeLocaleList(isolate, locales),
+                             Intl::CanonicalizeLocaleListJS(isolate, locales),
                              JSPluralRules);
 
   // 2. If options is undefined, then
diff --git a/src/runtime/runtime-intl.cc b/src/runtime/runtime-intl.cc
index a080c42c8519..47ec0ecd0384 100644
--- a/src/runtime/runtime-intl.cc
+++ b/src/runtime/runtime-intl.cc
@@ -94,8 +94,11 @@ RUNTIME_FUNCTION(Runtime_CanonicalizeLanguageTag) {
   DCHECK_EQ(1, args.length());
   CONVERT_ARG_HANDLE_CHECKED(Object, locale, 0);
 
-  RETURN_RESULT_OR_FAILURE(isolate,
-                           Intl::CanonicalizeLanguageTag(isolate, locale));
+  std::string canonicalized;
+  if (!Intl::CanonicalizeLanguageTag(isolate, locale).To(&canonicalized)) {
+    return ReadOnlyRoots(isolate).exception();
+  }
+  return *isolate->factory()->NewStringFromAsciiChecked(canonicalized.c_str());
 }
 
 RUNTIME_FUNCTION(Runtime_AvailableLocalesOf) {
@@ -112,7 +115,8 @@ RUNTIME_FUNCTION(Runtime_GetDefaultICULocale) {
   HandleScope scope(isolate);
 
   DCHECK_EQ(0, args.length());
-  return *Intl::DefaultLocale(isolate);
+  return *isolate->factory()->NewStringFromAsciiChecked(
+      Intl::DefaultLocale(isolate).c_str());
 }
 
 RUNTIME_FUNCTION(Runtime_IsWellFormedCurrencyCode) {
@@ -476,7 +480,7 @@ RUNTIME_FUNCTION(Runtime_StringToLowerCaseIntl) {
   DCHECK_EQ(args.length(), 1);
   CONVERT_ARG_HANDLE_CHECKED(String, s, 0);
   s = String::Flatten(isolate, s);
-  return ConvertToLower(s, isolate);
+  RETURN_RESULT_OR_FAILURE(isolate, ConvertToLower(s, isolate));
 }
 
 RUNTIME_FUNCTION(Runtime_StringToUpperCaseIntl) {
@@ -484,7 +488,7 @@ RUNTIME_FUNCTION(Runtime_StringToUpperCaseIntl) {
   DCHECK_EQ(args.length(), 1);
   CONVERT_ARG_HANDLE_CHECKED(String, s, 0);
   s = String::Flatten(isolate, s);
-  return ConvertToUpper(s, isolate);
+  RETURN_RESULT_OR_FAILURE(isolate, ConvertToUpper(s, isolate));
 }
 
 RUNTIME_FUNCTION(Runtime_DateCacheVersion) {