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

pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart

 // Copyright (c) 2014, 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.js_emitter.full_emitter;
 
 import 'dart:convert';
 import 'dart:collection' show HashMap;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 import 'package:js_runtime/shared/embedded_names.dart' show
     JsBuiltin,
     JsGetName;
 
 import '../headers.dart';
 
 import '../js_emitter.dart' hide Emitter;
 import '../js_emitter.dart' as js_emitter show Emitter;
 
 import '../model.dart';
 import '../program_builder/program_builder.dart';
 
 import '../../common.dart';
 
 import '../../constants/values.dart';
 
 import '../../deferred_load.dart' show OutputUnit;
 
+import '../../diagnostics/messages.dart' show MessageKind;
+
 import '../../diagnostics/spannable.dart' show
     NO_LOCATION_SPANNABLE;
 
 import '../../elements/elements.dart' show
     ConstructorBodyElement,
     ElementKind,
     FieldElement,
     ParameterElement,
     TypeVariableElement,
     MethodElement,
@@ skipping 22 matching lines @@
     JavaScriptBackend,
     JavaScriptConstantCompiler,
     Namer,
     RuntimeTypes,
     SetterName,
     Substitution,
     TypeCheck,
     TypeChecks,
     TypeVariableHandler;
 
-import '../../messages.dart' show MessageKind;
-
 import '../../util/characters.dart' show
     $$,
     $A,
     $HASH,
     $PERIOD,
     $Z,
     $a,
     $z;
 
 import '../../util/uri_extras.dart' show
@@ skipping 2036 matching lines @@
     for (Element element in compiler.enqueuer.codegen.newlyEnqueuedElements) {
       if (element.isInstanceMember) {
         cachedClassBuilders.remove(element.enclosingClass);
 
         nativeEmitter.cachedBuilders.remove(element.enclosingClass);
 
       }
     }
   }
 }