Make is checks work on JavaScriptFunction objects.

tests/html/js_test.dart

 // Copyright (c) 2013, 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.
 
 library jsTest;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:typed_data' show ByteBuffer, Int32List;
 import 'dart:indexed_db' show IdbFactory, KeyRange;
@@ skipping 176 matching lines @@
 }
 
 var someProto = { role: "proto" };
 var someObject = Object.create(someProto);
 someObject.role = "object";
 
 """;
   document.body.append(script);
 }
 
+typedef bool StringToBool(String s);
+
 // Some test are either causing other test to fail in IE9, or they are failing
 // for unknown reasons
 // useHtmlConfiguration+ImageData bug: dartbug.com/14355
 skipIE9_test(String description, t()) {
   if (Platform.supportsTypedData) {
     test(description, t);
   }
 }
 
 class Foo {
@@ skipping 708 matching lines @@
         if (Platform.supportsTypedData) {
           var list = context.callMethod('getNewInt32Array');
           print(list);
           expect(list is Int32List, isTrue);
           expect(list, orderedEquals([1, 2, 3, 4, 5, 6, 7, 8]));
         }
       });
 
     });
 
+    group('JavaScriptFunction', () {
+      test('is check', () {
+        var fn = (String s) => true;
+        var jsFn = allowInterop(fn);
+        expect(fn is StringToBool, isTrue);
+        expect(jsFn is StringToBool, isTrue);
+        expect(jsFn is Function, isTrue);
+        expect(jsFn is List, isFalse);
+      });
+    });
+
     group('Dart->JS', () {
 
       test('Date', () {
         context['o'] = new DateTime(1995, 12, 17);
         var dateType = context['Date'];
         expect(context.callMethod('isPropertyInstanceOf', ['o', dateType]),
             isTrue);
         context.deleteProperty('o');
       });
 
@@ skipping 70 matching lines @@
           // expect(context.callMethod('isPropertyInstanceOf', ['o', listType]),
           //    isTrue);
           context.deleteProperty('o');
         }
       });
 
     });
   });
 
 }