Add a mixin for assists similar to that for fixes

pkg/analysis_server/lib/src/services/correction/assist.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.
 
 import 'package:analysis_server/plugin/edit/assist/assist_core.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:analyzer/src/generated/source.dart';
+import 'package:analyzer_plugin/utilities/assist/assist.dart';
 
 /**
  * The implementation of [AssistContext].
  */
 class AssistContextImpl implements AssistContext {
   @override
   final AnalysisDriver analysisDriver;
 
   @override
   final Source source;
@@ skipping 96 matching lines @@
       const AssistKind('SURROUND_WITH_FOR_IN', 30, "Surround with 'for-in'");
   static const SURROUND_WITH_IF =
       const AssistKind('SURROUND_WITH_IF', 30, "Surround with 'if'");
   static const SURROUND_WITH_TRY_CATCH = const AssistKind(
       'SURROUND_WITH_TRY_CATCH', 30, "Surround with 'try-catch'");
   static const SURROUND_WITH_TRY_FINALLY = const AssistKind(
       'SURROUND_WITH_TRY_FINALLY', 30, "Surround with 'try-finally'");
   static const SURROUND_WITH_WHILE =
       const AssistKind('SURROUND_WITH_WHILE', 30, "Surround with 'while'");
 }