Re-apply change to move to plugin package

pkg/analysis_server/test/edit/assists_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.edit.assists;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/edit/edit_domain.dart';
 import 'package:analysis_server/src/plugin/server_plugin.dart';
 import 'package:analysis_server/src/protocol.dart';
-import 'package:analyzer/src/plugin/plugin_impl.dart';
+import 'package:plugin/manager.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart' hide ERROR;
 
 import '../analysis_abstract.dart';
 
 main() {
   groupSep = ' | ';
   defineReflectiveTests(AssistsTest);
 }
 
@@ skipping 84 matching lines @@
       if (change.message == message) {
         String resultCode =
             SourceEdit.applySequence(testCode, change.edits[0].edits);
         expect(resultCode, expectedCode);
         return;
       }
     }
     fail("Expected to find |$message| in\n" + changes.join('\n'));
   }
 }