Implement is-checks for closures.

sdk/lib/_internal/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.
 
 part of js_backend;
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
@@ skipping 1066 matching lines @@
     if (checkedClasses.contains(cls)) {
       emitIsTest(cls);
     }
 
     Set<Element> generated = new Set<Element>();
     // A class that defines a [:call:] method implicitly implements
     // [Function] and needs checks for all typedefs that are used in is-checks.
     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.
+        call = cls.lookupBackendMember(Compiler.CALL_OPERATOR_NAME);
+      }
       if (call != null) {
         generateInterfacesIsTests(compiler.functionClass,
                                   emitIsTest,
                                   generated);
         FunctionType callType = call.computeType(compiler);
         for (TypedefElement typedef in checkedTypedefs) {
           FunctionType typedefType =
               typedef.computeType(compiler).unalias(compiler);
           if (compiler.types.isSubtype(callType, typedefType)) {
             emitIsTest(typedef);
@@ skipping 919 matching lines @@
 """;
 const String HOOKS_API_USAGE = """
 // 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].
 """;