Remove more libraries directives from 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.plugin.edit.assist.assist_dart;
-
 import 'dart:async';
 
 import 'package:analysis_server/plugin/edit/assist/assist_core.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/src/dart/analysis/ast_provider_context.dart';
 import 'package:analyzer/src/dart/element/ast_provider.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
@@ skipping 99 matching lines @@
   /**
    * The start of the selection.
    */
   int get selectionOffset => _context.selectionOffset;
 
   /**
    * The source to get assists in.
    */
   Source get source => _context.source;
 }