Collect the types used in is-checks in the resolver phase.

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 eaf1f1a78bff1c7232a3aa4d7ef91bccaa950fdb..297fe85496916165131a65d6de37c5b80a8f6acd 100644
--- a/lib/compiler/implementation/js_backend/emitter.dart
+++ b/lib/compiler/implementation/js_backend/emitter.dart
@@ -574,7 +574,7 @@ function(collectedClasses) {
 
   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>());
@@ -586,7 +586,7 @@ function(collectedClasses) {
     for (Type 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);
       }