Added support for exports and re-exports.

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.
 
 class MessageKind {
   final String template;
   const MessageKind(this.template);
 
   static const GENERIC = const MessageKind('#{1}');
 
@@ skipping 30 matching lines @@
   static const CANNOT_RESOLVE_CONSTRUCTOR = const MessageKind(
       'cannot resolve constructor #{1}');
   static const CANNOT_RESOLVE_CONSTRUCTOR_FOR_IMPLICIT = const MessageKind(
       'cannot resolve constructor #{1} for implicit super call');
   static const CANNOT_RESOLVE_TYPE = const MessageKind(
       'cannot resolve type #{1}');
   static const DUPLICATE_DEFINITION = const MessageKind(
       'duplicate definition of #{1}');
   static const DUPLICATE_IMPORT = const MessageKind(
       'duplicate import of #{1}');
+  static const DUPLICATE_EXPORT = const MessageKind(
+      'duplicate export of #{1}');
   static const NOT_A_TYPE = const MessageKind(
       '#{1} is not a type');
   static const NOT_A_PREFIX = const MessageKind(
       '#{1} is not a prefix');
   static const NO_SUPER_IN_OBJECT = const MessageKind(
       "'Object' does not have a superclass");
   static const CANNOT_FIND_CONSTRUCTOR = const MessageKind(
       'cannot find constructor #{1}');
   static const CANNOT_FIND_CONSTRUCTOR2 = const MessageKind(
       'cannot find constructor #{1} in #{2}');
@@ skipping 290 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);
 }