Move the wire protocol support into the public API

pkg/analysis_server/test/analysis/notification_highlights_test2.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 test.analysis.notification.highlights2;
 
 import 'dart:async';
 
+import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/constants.dart';
-import 'package:analysis_server/src/protocol.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
 import '../analysis_abstract.dart';
 import '../utils.dart';
 
 main() {
   initializeTestEnvironment();
   defineReflectiveTests(AnalysisNotificationHighlightsTest);
   defineReflectiveTests(HighlightTypeTest);
@@ skipping 1145 matching lines @@
   void test_toString() {
     expect(HighlightRegionType.CLASS.toString(), 'HighlightRegionType.CLASS');
   }
 
   void test_valueOf_unknown() {
     expect(() {
       new HighlightRegionType('no-such-type');
     }, throws);
   }
 }