Add top-level driver and abstract plugin superclass

pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 import 'dart:isolate';
 
 import 'package:analyzer_plugin/channel/channel.dart';
 import 'package:analyzer_plugin/protocol/protocol.dart';
 
 /**
  * The object that allows a [ServerPlugin] to receive [Request]s and to return
- * both [Response]s and [Notification]s.
+ * both [Response]s and [Notification]s. It communicates with the analysis
+ * server by passing data to the server's main isolate.
  */
 class IsolateChannel implements PluginCommunicationChannel {
   /**
    * The port used to send notifications and responses to the server.
    */
   SendPort _sendPort;
 
   /**
    * The port used to receive requests from the server.
    */
@@ skipping 41 matching lines @@
   @override
   void sendNotification(Notification notification) {
     _sendPort.send(notification.toJson());
   }
 
   @override
   void sendResponse(Response response) {
     _sendPort.send(response.toJson());
   }
 }