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

pkg/compiler/lib/src/typechecker.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.typechecker;
 
 import 'common/tasks.dart' show
     CompilerTask;
 import 'compiler.dart' show
     Compiler,
     isPrivateName;
 import 'constants/expressions.dart';
 import 'constants/values.dart';
 import 'core_types.dart';
 import 'dart_types.dart';
 import 'diagnostics/invariant.dart' show
     invariant;
+import 'diagnostics/messages.dart';
 import 'diagnostics/spannable.dart' show
     Spannable;
 import 'elements/elements.dart' show
     AbstractFieldElement,
     AstElement,
     AsyncMarker,
     ClassElement,
     ConstructorElement,
     Element,
     Elements,
     EnumClassElement,
     ExecutableElement,
     FieldElement,
     FunctionElement,
     GetterElement,
     InitializingFormalElement,
     LibraryElement,
     Member,
     MemberSignature,
     Name,
     ParameterElement,
     PrivateName,
     PublicName,
     ResolvedAst,
     SetterElement,
     TypeDeclarationElement,
     TypedElement,
     TypedefElement,
     VariableElement;
-import 'messages.dart';
 import 'resolution/resolution.dart' show
     TreeElements;
 import 'resolution/class_members.dart' show
     MembersCreator;
 import 'tree/tree.dart';
 import 'util/util.dart' show
     Link,
     LinkBuilder;
 import '../compiler_new.dart' as api;
 
@@ skipping 1865 matching lines @@
 
   visitTypedef(Typedef node) {
     // Do not typecheck [Typedef] nodes.
   }
 
   visitNode(Node node) {
     compiler.internalError(node,
         'Unexpected node ${node.getObjectDescription()} in the type checker.');
   }
 }