Revert breaking change

pkg/analysis_server/test/services/correction/assist_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.correction.assist;
 
 import 'package:analysis_server/edit/assist/assist_core.dart';
 import 'package:analysis_server/src/plugin/server_plugin.dart';
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_server/src/services/correction/assist.dart';
 import 'package:analyzer/src/generated/source.dart';
-import 'package:plugin/manager.dart';
+import 'package:analyzer/src/plugin/plugin_impl.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
 import '../../abstract_single_unit.dart';
 
 main() {
   groupSep = ' | ';
   defineReflectiveTests(AssistProcessorTest);
 }
 
@@ skipping 3059 matching lines @@
       positions.add(new Position(testFile, offset));
     }
     return positions;
   }
 
   void _setStartEndSelection() {
     offset = findOffset('// start\n') + '// start\n'.length;
     length = findOffset('// end') - offset;
   }
 }