Reapply Issue 10911006.

lib/compiler/implementation/js_backend/emitter.dart

Index: lib/compiler/implementation/js_backend/emitter.dart
diff --git a/lib/compiler/implementation/js_backend/emitter.dart b/lib/compiler/implementation/js_backend/emitter.dart
index 68557551eabeaedf1eb02b1e7c558e8db6f76b43..541a3028751fd11cbbb66482f3cf3745b4aefa85 100644
--- a/lib/compiler/implementation/js_backend/emitter.dart
+++ b/lib/compiler/implementation/js_backend/emitter.dart
@@ -661,7 +661,7 @@ function(prototype, staticName, fieldName, getterName, lazyValue) {
 
   void generateTypeTests(ClassElement cls,
                          void generateTypeTest(ClassElement element)) {
-    if (compiler.codegenWorld.isChecks.contains(cls)) {
+    if (compiler.codegenWorld.checkedClasses.contains(cls)) {
       generateTypeTest(cls);
     }
     generateInterfacesIsTests(cls, generateTypeTest, new Set<Element>());
@@ -673,7 +673,7 @@ function(prototype, staticName, fieldName, getterName, lazyValue) {
     for (DartType interfaceType in cls.interfaces) {
       Element element = interfaceType.element;
       if (!alreadyGenerated.contains(element) &&
-          compiler.codegenWorld.isChecks.contains(element)) {
+          compiler.codegenWorld.checkedClasses.contains(element)) {
         alreadyGenerated.add(element);
         generateTypeTest(element);
       }