Move the wire protocol support into the public API

pkg/analysis_server/test/socket_server_test.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.socket.server;
 
 import 'dart:async';
 
+import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/plugin/server_plugin.dart';
-import 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_server/src/socket_server.dart';
 import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:analyzer/src/generated/sdk_io.dart';
 import 'package:plugin/manager.dart';
 import 'package:unittest/unittest.dart';
 
 import 'mocks.dart';
 import 'utils.dart';
 
 main() {
@@ skipping 108 matching lines @@
       new Future(throwException);
       return new Response(request.id);
     }
     throw 'mock request exception';
   }
 
   void throwException() {
     throw 'mock future exception';
   }
 }