Add optional message to assert in Dart2js.

sdk/lib/core/errors.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 dart.core;
 
 /**
  * Error objects thrown in the case of a program failure.
  *
  * An `Error` object represents a program failure that the programmer
@@ skipping 78 matching lines @@
 
   external static String _objectToString(Object object);
 
   external StackTrace get stackTrace;
 }
 
 /**
  * Error thrown by the runtime system when an assert statement fails.
  */
 class AssertionError extends Error {
+  AssertionError();
+  String toString() => "Assertion failed";
 }
 
 /**
  * Error thrown by the runtime system when a type assertion fails.
  */
 class TypeError extends AssertionError {
 }
 
 /**
  * Error thrown by the runtime system when a cast operation fails.
@@ skipping 453 matching lines @@
  * the first time it is read. If evaluating the initializer expression causes
  * another read of the variable, this error is thrown.
  */
 class CyclicInitializationError extends Error {
   final String variableName;
   CyclicInitializationError([this.variableName]);
   String toString() => variableName == null
       ? "Reading static variable during its initialization"
       : "Reading static variable '$variableName' during its initialization";
 }