Restructure the public API for server

pkg/analysis_server/lib/plugin/edit/assist/assist_dart.dart

 // Copyright (c) 2015, 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 analysis_server.edit.assist.assist_dart;
+library analysis_server.plugin.edit.assist.assist_dart;
 
-import 'package:analysis_server/edit/assist/assist_core.dart';
+import 'package:analysis_server/plugin/edit/assist/assist_core.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * An [AssistContributor] that can be used to contribute assists for Dart
  * files.
  *
  * Clients are expected to extend this class when implementing plugins.
  */
@@ skipping 18 matching lines @@
 
   /**
    * Return a list of assists for a location in the given [source]. The location
    * is specified by the [offset] and [length] of the selected region. The
    * [context] can be used to get additional information that is useful for
    * computing assists.
    */
   List<Assist> internalComputeAssists(
       CompilationUnit unit, int offset, int length);
 }