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 673c98822ac3d95ef585605ddb752b5020a8c739..d47816b639831e4d7fca6586de69595221e3ebe8 100644
--- a/lib/compiler/implementation/js_backend/native_emitter.dart
+++ b/lib/compiler/implementation/js_backend/native_emitter.dart
@@ -383,9 +383,10 @@ 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 (Type type in compiler.codegenWorld.isChecks) {
+      Element element = type.element;
+      if (!requiresNativeIsCheck(element)) continue;
+      String name = compiler.namer.operatorIs(element);
       objectProperties[name] = 'function() { return false; }';
     }
   }