Add support for javascript incompatibility warnings.

runtime/lib/errors_patch.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.
 
 patch class Error {
   /* patch */ static String _objectToString(Object object) {
     return Object._toString(object);
   }
 
   /* patch */ StackTrace get stackTrace => _stackTrace;
@@ skipping 316 matching lines @@
     return msg_buf.toString();
   }
 }
 
 class _JavascriptIntegerOverflowError extends Error {
   final Object _value;
 
   _JavascriptIntegerOverflowError(this._value);
   String toString() => "Javascript Integer Overflow: $_value";
 }
+
+class _JavascriptCompatibilityError extends Error {
+  final String _msg;
+
+  _JavascriptCompatibilityError(this._msg);
+  String toString() => "Javascript Compatibility Error: $_msg";
+}
+