Move the wire protocol support into the public API

pkg/analysis_server/test/analysis/notification_errors_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.notification_errors;
 
+import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/domain_analysis.dart';
-import 'package:analysis_server/src/protocol.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/services/lint.dart';
 import 'package:linter/src/linter.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
 import '../analysis_abstract.dart';
 import '../utils.dart';
 
 main() {
@@ skipping 127 matching lines @@
 ''');
     return waitForTasksFinished().then((_) {
       List<AnalysisError> errors = filesErrors[testFile];
       expect(errors, hasLength(1));
       AnalysisError error = errors[0];
       expect(error.severity, AnalysisErrorSeverity.WARNING);
       expect(error.type, AnalysisErrorType.STATIC_WARNING);
     });
   }
 }