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

lib/compiler/implementation/js_backend/native_emitter.dart

Index: lib/compiler/implementation/js_backend/native_emitter.dart
diff --git a/lib/compiler/implementation/js_backend/native_emitter.dart b/lib/compiler/implementation/js_backend/native_emitter.dart
index 8cd5c334bd0325ac0392609dfee93d1bcc9e0beb..b8b4c08d37283f101d8ad5faeef51a71f61d948b 100644
--- a/lib/compiler/implementation/js_backend/native_emitter.dart
+++ b/lib/compiler/implementation/js_backend/native_emitter.dart
@@ -383,9 +383,9 @@ function(cls, fields, methods) {
   }
 
   void emitIsChecks(Map<String, String> objectProperties) {
-    for (Element type in compiler.codegenWorld.isChecks) {
-      if (!requiresNativeIsCheck(type)) continue;
-      String name = compiler.namer.operatorIs(type);
+    for (Element element in compiler.codegenWorld.checkedClasses) {
+      if (!requiresNativeIsCheck(element)) continue;
+      String name = compiler.namer.operatorIs(element);
       objectProperties[name] = 'function() { return false; }';
     }
   }