Remove ClassWorld.subclassesOf and ClassWorld.subtypesOf.

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index e20069c1d525ac1cde096447efccf095a89b13af..25cf0513f525c78b864693108469dee9f9d50cd9 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -850,7 +850,7 @@ class JavaScriptBackend extends Backend {
     Iterable<MixinApplicationElement> uses = classWorld.mixinUsesOf(mixin);
     Set<ClassElement> result = null;
     for (MixinApplicationElement use in uses) {
-      Iterable<ClassElement> subclasses = classWorld.subclassesOf(use);
+      Iterable<ClassElement> subclasses = classWorld.strictSubclassesOf(use);
       for (ClassElement subclass in subclasses) {
         if (Elements.isNativeOrExtendsNative(subclass)) {
           if (result == null) result = new Set<ClassElement>();
@@ -2333,8 +2333,8 @@ class JavaScriptBackend extends Backend {
           }
         });
         // 4) all overriding members of subclasses/subtypes (should be resolved)
-        if (compiler.world.hasAnySubtype(cls)) {
-          for (ClassElement subcls in compiler.world.subtypesOf(cls)) {
+        if (compiler.world.hasAnyStrictSubtype(cls)) {
+          for (ClassElement subcls in compiler.world.strictSubtypesOf(cls)) {
             subcls.forEachClassMember((Member member) {
               if (memberNames.contains(member.name)) {
                 // TODO(20993): find out why this assertion fails.