Prepare for improved error recovery from compile-time errors.

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);
   }
@@ skipping 412 matching lines @@
 ///
 /// Note: in strong mode, this is a compile-time error and this class becomes
 /// obsolete.
 class _DuplicatedFieldInitializerError extends Error {
   final String _name;
 
   _DuplicatedFieldInitializerError(this._name);
 
   toString() => "Error: field '$_name' is already initialized.";
 }
+
+@patch
+class _ConstantExpressionError {
+  @patch
+  _throw(error) => throw error;
+}