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

tests/compiler/dart2js/unparser2_test.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.
 
 import "package:expect/expect.dart";
 import "package:compiler/src/scanner/scannerlib.dart";
 import "package:compiler/src/tree/tree.dart";
 
-import "package:compiler/src/diagnostic_listener.dart";
+import "package:compiler/src/diagnostics/diagnostic_listener.dart";
 import "package:compiler/src/elements/elements.dart"
     show CompilationUnitElement,
          LibraryElement;
 import "package:compiler/src/elements/modelx.dart"
     show CompilationUnitElementX,
          LibraryElementX;
 import "package:compiler/src/script.dart";
 
 main() {
   testClassDef();
@@ skipping 85 matching lines @@
     messages.add(reason);
     throw reason;
   }
 
   void log(message) {
     messages.add(message);
   }
 
   noSuchMethod(Invocation invocation) => throw 'unsupported operation';
 }