minor code cleanup

lib/runtime/dart_core.js

 // Copyright (c) 2015, 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.
 
 var dart_core;
 (function (dart_core) {
   var Object = (function () {
     var constructor = function Object() {};
 
     constructor.prototype.toString = function toString() {
@@ skipping 29 matching lines @@
     return NoSuchMethodError;
   })();
   dart_core.NoSuchMethodError = NoSuchMethodError;
 
   // Class UnimplementedError
   var UnimplementedError = (function () {
     // TODO(vsm): Implement.
     function UnimplementedError(message) {
       this.message = (message != void 0) ? message : null;
     }
-    return NoSuchMethodError;
+    return UnimplementedError;
   })();
   dart_core.UnimplementedError = UnimplementedError;
 })(dart_core || (dart_core = {}));