Move diagnostic_listener.dart and messages.dart to the diagnostics folder

tests/compiler/dart2js/compiler_helper.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.
 
 library compiler_helper;
 
 import 'dart:async';
 import "package:expect/expect.dart";
 
 import 'package:compiler/compiler.dart' as api;
 
 import 'package:compiler/src/elements/elements.dart'
        as lego;
 export 'package:compiler/src/elements/elements.dart';
 
 import 'package:compiler/src/js_backend/js_backend.dart'
        as js;
 
 import 'package:compiler/src/common/codegen.dart';
 import 'package:compiler/src/common/resolution.dart';
 
+export 'package:compiler/src/diagnostics/messages.dart';
 export 'package:compiler/src/diagnostics/source_span.dart';
 export 'package:compiler/src/diagnostics/spannable.dart';
-export 'package:compiler/src/messages.dart';
 
 import 'package:compiler/src/types/types.dart'
        as types;
 export 'package:compiler/src/types/types.dart'
        show TypeMask;
 
 import 'package:compiler/src/util/util.dart';
 export 'package:compiler/src/util/util.dart';
 
 import 'package:compiler/src/compiler.dart'
@@ skipping 223 matching lines @@
     regexp = regexp.replaceAll(xRe, '(?:$anyIdentifier)');
     final spaceRe = new RegExp('\\s+');
     regexp = regexp.replaceAll(spaceRe, '(?:\\s*)');
     if (shouldMatch) {
       Expect.isTrue(new RegExp(regexp).hasMatch(generated));
     } else {
       Expect.isFalse(new RegExp(regexp).hasMatch(generated));
     }
   });
 }