Make the VM's dart:core and dart:async library patches clean.

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 static String _stringToSafeString(String string) {
@@ skipping 85 matching lines @@
   final int _line;
   final int _column;
   final String _errorMsg;
 }
 
 @patch class FallThroughError {
   FallThroughError._create(this._url, this._line);
 
   static _throwNew(int case_clause_pos) native "FallThroughError_throwNew";
 
+  @patch
   String toString() {
     return "'$_url': Switch case fall-through at line $_line.";
   }
 
   // These new fields cannot be declared final, because a constructor exists
   // in the original version of this patched class.
   String _url;
   int _line;
 }
 
@@ skipping 240 matching lines @@
     return msg_buf.toString();
   }
 }
 
 
 class _CompileTimeError extends Error {
   final String _errorMsg;
   _CompileTimeError(this._errorMsg);
   String toString() => _errorMsg;
 }