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

tests/compiler/dart2js/library_resolution_test.dart

 // Copyright (c) 2014, 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.
 
 /// Check that relative URIs are resolved against the canonical name of a
 /// library. This only matters for dart:-libraries, so this test mocks up two
 /// dart:-libraries.
 
 import "dart:io";
 
 import "dart:async";
 
 import "memory_source_file_helper.dart";
 
 import "package:async_helper/async_helper.dart";
 
 import 'package:expect/expect.dart' show
     Expect;
 
+import 'package:compiler/src/diagnostics/messages.dart' show
+    MessageKind,
+    MessageTemplate;
+
 import 'package:compiler/src/elements/elements.dart' show
     LibraryElement;
 
-import 'package:compiler/src/messages.dart' show
-    MessageKind,
-    MessageTemplate;
-
 import 'package:compiler/src/null_compiler_output.dart' show
     NullCompilerOutput;
 
 import 'package:compiler/src/old_to_new_api.dart' show
     LegacyCompilerDiagnostics,
     LegacyCompilerInput;
 
 import 'package:sdk_library_metadata/libraries.dart' show
     DART2JS_PLATFORM,
     LibraryInfo;
@@ skipping 86 matching lines @@
   asyncStart();
   compiler.libraryLoader.loadLibrary(Uri.parse("dart:m_o_c_k_1"))
       .then(checkLibrary)
       .then(asyncSuccess);
 }
 
 const Map MEMORY_SOURCE_FILES = const {
   "mock1.dart": "library mock1; import 'mock2.dart';",
   "mock2.dart": "library mock2;",
 };