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

lib/compiler/implementation/enqueue.dart

Index: lib/compiler/implementation/enqueue.dart
diff --git a/lib/compiler/implementation/enqueue.dart b/lib/compiler/implementation/enqueue.dart
index 1172b5f3bdb4a3b9802cc6073eeb008beea43d56..930e75ce0267650b559e6fd2ef5e683fc470287d 100644
--- a/lib/compiler/implementation/enqueue.dart
+++ b/lib/compiler/implementation/enqueue.dart
@@ -334,9 +334,8 @@ class Enqueuer {
     });
   }
 
-  // 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)) {