Move the wire protocol support into the public API

pkg/analysis_server/test/analysis/set_priority_files_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.analysis.set_priority_files;
 
+import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/domain_analysis.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 '../mocks.dart';
 import '../utils.dart';
 
 main() {
   initializeTestEnvironment();
   defineReflectiveTests(SetPriorityFilesTest);
@@ skipping 34 matching lines @@
     expect(response.error, isNotNull);
     expect(response.error.code, RequestErrorCode.UNANALYZED_PRIORITY_FILES);
   }
 
   _setPriorityFile(String file) async {
     Request request =
         new AnalysisSetPriorityFilesParams(<String>[file]).toRequest('0');
     return await serverChannel.sendRequest(request);
   }
 }