Add special case for prototype in AmbiguateProperties

Closes #2885
Closes #2888

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=193225397

src/com/google/javascript/jscomp/AmbiguateProperties.java

@@ -87,7 +87,7 @@ class AmbiguateProperties implements CompilerPass {
   private final Map<String, Property> propertyMap = new HashMap<>();
 
   /** Property names that don't get renamed */
-  private final Set<String> externedNames;
+  private final ImmutableSet<String> externedNames;
 
   /** Names to which properties shouldn't be renamed, to avoid name conflicts */
   private final Set<String> quotedNames = new HashSet<>();
@@ -140,7 +140,11 @@ public int compare(Property p1, Property p2) {
         .addAllTypeMismatches(compiler.getImplicitInterfaceUses())
         .build();
 
-    this.externedNames = compiler.getExternProperties();
+    this.externedNames =
+        ImmutableSet.<String>builder()
+            .add("prototype")
+            .addAll(compiler.getExternProperties())
+            .build();
   }
 
   static AmbiguateProperties makePassForTesting(

test/com/google/javascript/jscomp/AmbiguatePropertiesTest.java

@@ -1144,4 +1144,22 @@ public void testDontInvalidateParameterizedObjectTypes() {
     this.mode = TypeInferenceMode.NTI_ONLY;
     test(js, output);
   }
+
+  public void testDontRenamePrototypeWithoutExterns() {
+    String js = lines(
+      "/** @interface */",
+      "function Foo() {}",
+      "/** @return {!Foo} */",
+      "Foo.prototype.foo = function() {};");
+
+    String output = lines(
+      "/** @interface */",
+      "function Foo() {}",
+      "/** @return {!Foo} */",
+      "Foo.prototype.a = function() {};");
+
+    // NTI reqires at least the MINIMAL_EXTERNS.
+    this.mode = TypeInferenceMode.OTI_ONLY;
+    test(externs(""), srcs(js), expected(output));
+  }
 }