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

pkg/compiler/lib/src/mirrors_used.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.mirrors_used;
 
 import 'common/tasks.dart' show
     CompilerTask;
 import 'compile_time_constants.dart' show
     ConstantCompiler;
 import 'compiler.dart' show
     Compiler;
 import 'constants/expressions.dart';
 import 'constants/values.dart' show
     ConstantValue,
     ConstructedConstantValue,
     ListConstantValue,
     StringConstantValue,
     TypeConstantValue;
 import 'dart_types.dart' show
     DartType,
     InterfaceType,
     TypeKind;
+import 'diagnostics/messages.dart' show
+    MessageKind;
 import 'diagnostics/spannable.dart' show
     Spannable;
 import 'elements/elements.dart' show
     ClassElement,
     Element,
     LibraryElement,
     MetadataAnnotation,
     ScopeContainerElement,
     VariableElement;
-import 'messages.dart' show
-    MessageKind;
 import 'resolution/resolution.dart' show
     TreeElements;
 import 'tree/tree.dart' show
     Import,
     LibraryTag,
     NamedArgument,
     NewExpression,
     Node;
 
 /**
@@ skipping 541 matching lines @@
       // @MirrorsUsed(targets: fisk)
       //                       ^^^^
       //
       // Instead of saying 'fisk' should pretty print the problematic constant
       // value.
       return spannable;
     }
     return node;
   }
 }