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

pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart

 // Copyright (c) 2015, 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.js_emitter.startup_emitter.model_emitter;
 
 import 'dart:convert' show JsonEncoder;
 
 import '../../common.dart';
 
 import '../../constants/values.dart' show ConstantValue, FunctionConstantValue;
 import '../../compiler.dart' show Compiler;
+import '../../diagnostics/messages.dart' show
+    MessageKind;
+
 import '../../elements/elements.dart' show ClassElement, FunctionElement;
 import '../../hash/sha1.dart' show Hasher;
 
 import '../../io/code_output.dart';
 
 import '../../io/line_column_provider.dart' show
     LineColumnCollector,
     LineColumnProvider;
 
 import '../../io/source_map_builder.dart' show
     SourceMapBuilder;
 
 import '../../js/js.dart' as js;
 import '../../js_backend/js_backend.dart' show
     JavaScriptBackend,
     Namer,
     ConstantEmitter;
 
-import '../../messages.dart' show
-    MessageKind;
-
 import '../../diagnostics/spannable.dart' show
     NO_LOCATION_SPANNABLE;
 
 import '../../util/uri_extras.dart' show
     relativize;
 
 import '../headers.dart';
 import '../js_emitter.dart' show AstContainer, NativeEmitter;
 
 import 'package:js_runtime/shared/embedded_names.dart' show
@@ skipping 356 matching lines @@
     // Json does not support comments, so we embed the explanation in the
     // data.
     mapping["_comment"] = "This mapping shows which compiled `.js` files are "
         "needed for a given deferred library import.";
     mapping.addAll(compiler.deferredLoadTask.computeDeferredMap());
     compiler.outputProvider(compiler.deferredMapUri.path, 'deferred_map')
       ..add(const JsonEncoder.withIndent("  ").convert(mapping))
       ..close();
   }
 }