Move the wire protocol support into the public API

pkg/analysis_server/test/services/completion/prefixed_element_contributor_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.services.completion.invocation;
 
 import 'dart:async';
 
-import 'package:analysis_server/src/protocol.dart';
+import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
 import 'package:analysis_server/src/services/completion/prefixed_element_contributor.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
 import '../../utils.dart';
 import 'completion_test_util.dart';
 
 main() {
   initializeTestEnvironment();
@@ skipping 646 matching lines @@
       assertNotSuggested('ms2');
       assertSuggestInvocationMethod('m', 'C2', null,
           relevance: DART_RELEVANCE_HIGH);
       assertNotSuggested('fi3');
       assertNotSuggested('fs3');
       assertNotSuggested('mi3');
       assertNotSuggested('ms3');
     });
   }
 }