Add 'part of' in dart2js code.

lib/compiler/implementation/warnings.dart

 // Copyright (c) 2011, 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 dart2js;
+
 class MessageKind {
   final String template;
   const MessageKind(this.template);
 
   static const GENERIC = const MessageKind('#{1}');
 
   static const NOT_ASSIGNABLE = const MessageKind(
       '#{2} is not assignable to #{1}');
   static const VOID_EXPRESSION = const MessageKind(
       'expression does not yield a value');
@@ skipping 389 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }