Make union types more efficient by eliminating unnecessary redundancy.

This change modifies union types to always leave structural subtypes in
the union.  This can result in additional types errors if the previously
collapsed type provides a definition for a property or if the collapsed
type was @struct (as any @struct member of the union causes the member
to be considered @struct).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=242546590

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

@@ -135,7 +135,7 @@ static final MaybeBoxedIterableOrAsyncIterable maybeBoxIterableOrAsyncIterable(
     // Note: we don't just use JSType.autobox() here because that removes null and undefined.
     // We want to keep null and undefined around because they should cause a mismatch.
     if (type.isUnionType()) {
-      for (JSType alt : type.toMaybeUnionType().getAlternatesWithoutStructuralTyping()) {
+      for (JSType alt : type.toMaybeUnionType().getAlternates()) {
         alt = alt.isBoxableScalar() ? alt.autoboxesTo() : alt;
         boolean isIterable = alt.isSubtypeOf(typeRegistry.getNativeType(ITERABLE_TYPE));
         boolean isAsyncIterable = alt.isSubtypeOf(typeRegistry.getNativeType(ASYNC_ITERABLE_TYPE));

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

@@ -74,7 +74,7 @@ static final JSType getResolvedType(JSTypeRegistry registry, JSType type) {
 
     if (type.isUnionType()) {
       UnionTypeBuilder unionTypeBuilder = UnionTypeBuilder.create(registry);
-      for (JSType alternate : type.toMaybeUnionType().getAlternatesWithoutStructuralTyping()) {
+      for (JSType alternate : type.toMaybeUnionType().getAlternates()) {
         unionTypeBuilder.addAlternate(getResolvedType(registry, alternate));
       }
       return unionTypeBuilder.build();

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

@@ -283,7 +283,7 @@ boolean expectAutoboxesToIterable(Node n, JSType type, String msg) {
     // Note: we don't just use JSType.autobox() here because that removes null and undefined.
     // We want to keep null and undefined around.
     if (type.isUnionType()) {
-      for (JSType alt : type.toMaybeUnionType().getAlternatesWithoutStructuralTyping()) {
+      for (JSType alt : type.toMaybeUnionType().getAlternates()) {
         alt = alt.isBoxableScalar() ? alt.autoboxesTo() : alt;
         if (!alt.isSubtypeOf(getNativeType(ITERABLE_TYPE))) {
           mismatch(n, msg, type, ITERABLE_TYPE);

src/com/google/javascript/rhino/jstype/JSType.java

@@ -1448,9 +1448,7 @@ public TypePair getTypesUnderShallowInequality(JSType that) {
   }
 
   public Iterable<JSType> getUnionMembers() {
-    return isUnionType()
-        ? this.toMaybeUnionType().getAlternatesWithoutStructuralTyping()
-        : null;
+    return isUnionType() ? this.toMaybeUnionType().getAlternates() : null;
   }
 
   /**
@@ -1558,10 +1556,9 @@ static boolean isSubtypeHelper(JSType thisType, JSType thatType,
     if (thatType.isUnionType()) {
       UnionType union = thatType.toMaybeUnionType();
       // use an indexed for-loop to avoid allocations
-      ImmutableList<JSType> alternatesWithoutStucturalTyping =
-          union.getAlternatesWithoutStructuralTyping();
-      for (int i = 0; i < alternatesWithoutStucturalTyping.size(); i++) {
-        JSType element = alternatesWithoutStucturalTyping.get(i);
+      ImmutableList<JSType> alternates = union.getAlternates();
+      for (int i = 0; i < alternates.size(); i++) {
+        JSType element = alternates.get(i);
         if (thisType.isSubtype(element, implicitImplCache, subtypingMode)) {
           return true;
         }