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 156 matching lines @@
 
   void processInstantiatedClassMember(Element member) {
     if (universe.generatedCode.containsKey(member)) return;
     if (resolvedElements[member] !== null) return;
     if (!member.isInstanceMember()) return;
 
     if (member.kind === ElementKind.FIELD) {
       universe.instantiatedClassInstanceFields.add(member.name);
       if (compiler.enableTypeAssertions) {
         DartType type = member.computeType(compiler);
-        registerIsCheck(type.element);
+        registerIsCheck(type);
         SourceString helper = compiler.backend.getCheckedModeHelper(type);
         if (helper != null) {
           Element helperElement = compiler.findHelper(helper);
           registerStaticUse(helperElement);
         }
       }
       return;
     }
 
     String memberName = member.name.slowToString();
@@ skipping 153 matching lines @@
                            LibraryElement library,
                            DartType 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(DartType type) {
+    universe.isChecks.add(type);
   }
 
   void forEach(f(WorkItem work)) {
     while (!queue.isEmpty()) {
       f(queue.removeLast()); // TODO(kasperl): Why isn't this removeFirst?
     }
   }
 }