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

pkg/dart2js_incremental/lib/library_updater.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.
 
 library dart2js_incremental.library_updater;
 
 import 'dart:async' show
     Future;
 
 import 'package:compiler/compiler.dart' as api;
 
 import 'package:compiler/src/compiler.dart' show
     Compiler;
 
+import 'package:compiler/src/diagnostics/messages.dart' show
+    MessageKind;
+
 import 'package:compiler/src/elements/elements.dart' show
     ClassElement,
     CompilationUnitElement,
     Element,
     FunctionElement,
     LibraryElement,
     STATE_NOT_STARTED,
     ScopeContainerElement;
 
 import 'package:compiler/src/enqueue.dart' show
     EnqueueTask;
 
-import 'package:compiler/src/messages.dart' show
-    MessageKind;
-
 import 'package:compiler/src/scanner/scannerlib.dart' show
     EOF_TOKEN,
     Listener,
     NodeListener,
     Parser,
     PartialClassElement,
     PartialElement,
     PartialFieldList,
     PartialFunctionElement,
     Scanner,
@@ skipping 1464 matching lines @@
         .buildFieldsHackForIncrementalCompilation(classElement);
     // TODO(ahe): Rewrite for new emitter.
     ClassBuilder builder = new ClassBuilder(classElement, namer);
     classEmitter.emitFields(cls, builder);
     return builder.fields;
   }
 }
 
 // TODO(ahe): Remove this method.
 NO_WARN(x) => x;