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

lib/compiler/implementation/enqueue.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class EnqueueTask extends CompilerTask {
   final Enqueuer codegen;
   final Enqueuer resolution;
 
   String get name => 'Enqueue';
 
@@ skipping 316 matching lines @@
                            LibraryElement library,
                            Type type) {
     task.measure(() {
       Selector setter = new Selector.setter(setterName, library);
       registerNewSelector(setterName,
                           new TypedSelector(type, setter),
                           universe.fieldSetters);
     });
   }
 
-  // TODO(ngeoffray): This should get a type.
-  void registerIsCheck(Element element) {
-    universe.isChecks.add(element);
+  void registerIsCheck(Type type) {
+    universe.isChecks.add(type);
   }
 
   void forEach(f(WorkItem work)) {
     while (!queue.isEmpty()) {
       f(queue.removeLast()); // TODO(kasperl): Why isn't this removeFirst?
     }
   }
 }