Implement is-checks for closures.

sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
index 7b7df7049c2abed7692025b1268dc227ab6a7322..91f40b445b4d47657722adb0a94a351192ba6fff 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
@@ -1056,6 +1056,15 @@ $lazyInitializerLogic
     if (checkedClasses.contains(compiler.functionClass) ||
         !checkedTypedefs.isEmpty) {
       FunctionElement call = cls.lookupLocalMember(Compiler.CALL_OPERATOR_NAME);
+      if (call == null) {
+        // If [cls] is a closure, it has a synthetic call operator method.

[ngeoffray, 2012/12/12 13:28:02]

Please add a cls.lookupBackendMember.

+        for (Element element in cls.backendMembers) {
+          if (element.name == Compiler.CALL_OPERATOR_NAME) {
+            call = element;
+            break;
+          }
+        }
+      }
       if (call != null) {
         generateInterfacesIsTests(compiler.functionClass,
                                   emitIsTest,