Change UnionType to hold a List instead of a Collection for the alter…

…nates 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=139602401

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

@@ -44,13 +44,14 @@
 import com.google.common.base.Preconditions;
 import com.google.common.base.Predicate;
 import com.google.common.collect.HashBasedTable;
-import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
 import com.google.javascript.rhino.ErrorReporter;
 import com.google.javascript.rhino.JSDocInfo;
 import com.google.javascript.rhino.TypeI;
 import java.io.Serializable;
 import java.util.Comparator;
 import java.util.IdentityHashMap;
+import java.util.List;
 
 /**
  * Represents JavaScript value types.<p>
@@ -78,8 +79,8 @@ public abstract class JSType implements TypeI, Serializable {
   private static final CanCastToVisitor CAN_CAST_TO_VISITOR =
       new CanCastToVisitor();
 
-  private static final ImmutableList<String> COVARIANT_TYPES =
-      ImmutableList.of("Object", "IArrayLike", "Array");
+  private static final ImmutableSet<String> COVARIANT_TYPES =
+      ImmutableSet.of("Object", "IArrayLike", "Array");
 
   /**
    * Total ordering on types based on their textual representation.
@@ -302,7 +303,9 @@ 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()) {
+      List<JSType> alternates = this.toMaybeUnionType().getAlternates();
+      for (int i = 0; i < alternates.size(); i++) {
+        JSType alternate = alternates.get(i);
         if (alternate.isSubtype(arrayType)) {
           return true;
         }
@@ -1126,7 +1129,9 @@ static JSType filterNoResolvedType(JSType type) {
     } else if (type.isUnionType()) {
       UnionType unionType = type.toMaybeUnionType();
       boolean needsFiltering = false;
-      for (JSType alt : unionType.getAlternates()) {
+      List<JSType> alternates = unionType.getAlternates();
+      for (int i = 0; i < alternates.size(); i++) {
+        JSType alt = alternates.get(i);
         if (alt.isNoResolvedType()) {
           needsFiltering = true;
           break;
@@ -1406,8 +1411,10 @@ static boolean isSubtypeHelper(JSType thisType, JSType thatType,
     // unions
     if (thatType.isUnionType()) {
       UnionType union = thatType.toMaybeUnionType();
-      for (JSType element : union.alternatesWithoutStucturalTyping) {
-        if (thisType.isSubtype(element, implicitImplCache, subtypingMode)) {
+      List<JSType> alternates = union.alternatesWithoutStucturalTyping;
+      // use indexed iteration to avoid iterator allocations
+      for (int i = 0; i < alternates.size(); i++) {
+        if (thisType.isSubtype(alternates.get(i), implicitImplCache, subtypingMode)) {
           return true;
         }
       }