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

pkg/compiler/lib/src/library_loader.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 dart2js.library_loader;
 
 import 'dart:async';
 
 import 'common/tasks.dart' show
     CompilerTask;
 import 'compiler.dart' show
     Compiler;
-import 'diagnostic_listener.dart';
+import 'diagnostics/diagnostic_listener.dart';
 import 'diagnostics/invariant.dart' show
     invariant;
+import 'diagnostics/messages.dart' show
+    MessageKind;
 import 'elements/elements.dart' show
     CompilationUnitElement,
     Element,
     LibraryElement,
     PrefixElement;
 import 'elements/modelx.dart' show
     CompilationUnitElementX,
     DeferredLoaderGetterElementX,
     ErroneousElementX,
     LibraryElementX,
     PrefixElementX;
-import 'messages.dart' show
-    MessageKind;
 import 'native/native.dart' as native;
 import 'script.dart';
 import 'tree/tree.dart';
 import 'util/util.dart' show
     Link,
     LinkBuilder;
 
 /**
  * [CompilerTask] for loading libraries and setting up the import/export scopes.
  *
@@ skipping 1175 matching lines @@
         }
         suffixes.add(const Link<Uri>().prepend(canonicalUri));
       }
       suffixChainMap[library] = suffixes;
       return;
     }
 
     computeSuffixes(rootLibrary, const Link<Uri>());
   }
 }