Change UnionType to hold an ImmutableList instead of a Collection for…

… the alternates and

switch most accessors to do indexed iteration.

heap profiling showed that the iteration in JsType.checkEquivalenceHelper was
allocating lots of iterators. This is an easy, if slightly annoying, way to
make these operations a little cheaper.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=140442040

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

@@ -302,7 +302,10 @@ public boolean containsArray() {
     // Check if this is a union that contains an array
     if (this.isUnionType()) {
       JSType arrayType = registry.getNativeType(JSTypeNative.ARRAY_TYPE);
-      for (JSType alternate : this.toMaybeUnionType().getAlternates()) {
+      // use an indexed loop to avoid allocations
+      ImmutableList<JSType> alternatesList = this.toMaybeUnionType().getAlternatesList();
+      for (int i = 0; i < alternatesList.size(); i++) {
+        JSType alternate = alternatesList.get(i);
         if (alternate.isSubtype(arrayType)) {
           return true;
         }
@@ -1136,7 +1139,9 @@ static JSType filterNoResolvedType(JSType type) {
       if (needsFiltering) {
         UnionTypeBuilder builder = new UnionTypeBuilder(type.registry);
         builder.addAlternate(type.getNativeType(JSTypeNative.NO_RESOLVED_TYPE));
-        for (JSType alt : unionType.getAlternates()) {
+        ImmutableList<JSType> alternatesList = unionType.getAlternatesList();
+        for (int i = 0; i < alternatesList.size(); i++) {
+          JSType alt = alternatesList.get(i);
           if (!alt.isNoResolvedType()) {
             builder.addAlternate(alt);
           }
@@ -1406,7 +1411,11 @@ static boolean isSubtypeHelper(JSType thisType, JSType thatType,
     // unions
     if (thatType.isUnionType()) {
       UnionType union = thatType.toMaybeUnionType();
-      for (JSType element : union.alternatesWithoutStucturalTyping) {
+      // use an indexed for-loop to avoid allocations
+      ImmutableList<JSType> alternatesWithoutStucturalTyping =
+          union.alternatesWithoutStucturalTyping;
+      for (int i = 0; i < alternatesWithoutStucturalTyping.size(); i++) {
+        JSType element = alternatesWithoutStucturalTyping.get(i);
         if (thisType.isSubtype(element, implicitImplCache, subtypingMode)) {
           return true;
         }