Fix bug in dart2js spotted by the change:

lib/compiler/implementation/js_backend/emitter.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.
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
 class ClosureInvocationElement extends FunctionElement {
   ClosureInvocationElement(SourceString name,
@@ skipping 680 matching lines @@
                                  void generateTypeTest(ClassElement element),
                                  Set<Element> alreadyGenerated) {
     for (DartType interfaceType in cls.interfaces) {
       Element element = interfaceType.element;
       if (!alreadyGenerated.contains(element) &&
           compiler.codegenWorld.checkedClasses.contains(element)) {
         alreadyGenerated.add(element);
         generateTypeTest(element);
       }
       generateInterfacesIsTests(element, generateTypeTest, alreadyGenerated);
+
+      // Since [element] is implemented by [cls], we need to also emit
+      // is checks for the superclass and its supertypes.
+      ClassElement superclass = element.superclass;
+      if (!alreadyGenerated.contains(superclass) &&
+          compiler.codegenWorld.checkedClasses.contains(superclass)) {
+        alreadyGenerated.add(superclass);
+        generateTypeTest(superclass);
+      }
+      generateInterfacesIsTests(superclass, generateTypeTest, alreadyGenerated);
     }
   }
 
   void emitClasses(CodeBuffer buffer) {
     Set<ClassElement> instantiatedClasses =
         compiler.codegenWorld.instantiatedClasses;
     Set<ClassElement> neededClasses =
         new Set<ClassElement>.from(instantiatedClasses);
     for (ClassElement element in instantiatedClasses) {
       for (ClassElement superclass = element.superclass;
@@ skipping 626 matching lines @@
 const String HOOKS_API_USAGE = """
 // Generated by dart2js, the Dart to JavaScript compiler.
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called
 //                        instead of the Dart [print] method.
 // dartMainRunner(main) - if this function is defined, the Dart [main]
 //                        method will not be invoked directly.
 //                        Instead, a closure that will invoke [main] is
 //                        passed to [dartMainRunner].
 """;