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

tests/compiler/dart2js/memory_compiler.dart

 // Copyright (c) 2013, 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 dart2js.test.memory_compiler;
 
 import 'dart:async';
 
 import 'package:compiler/compiler.dart' show
     DiagnosticHandler;
 import 'package:compiler/compiler_new.dart' show
     CompilationResult,
     CompilerDiagnostics,
     CompilerOutput,
     Diagnostic,
     PackagesDiscoveryProvider;
-import 'package:compiler/src/messages.dart' show
+import 'package:compiler/src/diagnostics/messages.dart' show
     Message;
 import 'package:compiler/src/mirrors/source_mirrors.dart';
 import 'package:compiler/src/mirrors/analyze.dart';
 import 'package:compiler/src/null_compiler_output.dart' show
     NullCompilerOutput;
 import 'package:compiler/src/library_loader.dart' show
     LoadedLibraries;
 
 import 'memory_source_file_helper.dart';
 
@@ skipping 295 matching lines @@
       createDiagnosticHandler(diagnosticHandler, provider, showDiagnostics);
 
   List<Uri> libraries = <Uri>[];
   memorySourceFiles.forEach((String path, _) {
     libraries.add(new Uri(scheme: 'memory', path: path));
   });
 
   return analyze(libraries, libraryRoot, packageRoot,
                  provider, handler, options);
 }