fix assert with message

sdk/lib/_internal/js_runtime/lib/js_helper.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 _js_helper;
 
 import 'dart:_js_embedded_names' show
     DEFERRED_LIBRARY_URIS,
     DEFERRED_LIBRARY_HASHES,
     GET_TYPE_FROM_NAME,
@@ skipping 4031 matching lines @@
 
 void badMain() {
   throw new MainError("'main' is not a function.");
 }
 
 void mainHasTooManyParameters() {
   throw new MainError("'main' expects too many parameters.");
 }
 
 class _AssertionError extends AssertionError {
-  final _message;
-  _AssertionError(this._message);
+  _AssertionError(Object message) : super(message);
 
-  String toString() => "Assertion failed: " + Error.safeToString(_message);
+  String toString() => "Assertion failed: " + Error.safeToString(message);
 }
 
 
 // [_UnreachableError] is a separate class because we always resolve
 // [assertUnreachable] and want to reduce the impact of resolving possibly
 // unneeded code.
 class _UnreachableError extends AssertionError {
   _UnreachableError();
   String toString() => "Assertion failed: Reached dead code";
 }
 
 @NoInline()
 void assertUnreachable() {
   throw new _UnreachableError();
 }