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

pkg/compiler/lib/src/closure.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 closureToClassMapper;
 
 import 'common/tasks.dart' show
     CompilerTask;
 import 'compiler.dart' show
     Compiler;
 import 'constants/expressions.dart';
 import 'dart_types.dart';
-import 'diagnostic_listener.dart';
+import 'diagnostics/diagnostic_listener.dart';
 import 'diagnostics/spannable.dart' show
     SpannableAssertionFailure;
 import 'elements/elements.dart';
 import 'elements/modelx.dart' show
     BaseFunctionElementX,
     ClassElementX,
     ElementX,
     LocalFunctionElementX;
 import 'elements/visitor.dart' show ElementVisitor;
 import 'js_backend/js_backend.dart' show JavaScriptBackend;
@@ skipping 1095 matching lines @@
 
   String get name => typeVariable.name;
 
   int get hashCode => typeVariable.hashCode;
 
   bool operator ==(other) {
     if (other is! TypeVariableLocal) return false;
     return typeVariable == other.typeVariable;
   }
 }