Add a channel to communication with server

pkg/analyzer_plugin/lib/channel/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 '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.
+ *
+ * Clients may not extend, implement or mix-in this class.
+ */
+abstract class PluginCommunicationChannel {
+  /**
+   * Close the communication channel.
+   */
+  void close();
+
+  /**
+   * Listen to the channel for requests. If a request is received, invoke the
+   * [onRequest] function. If an error is encountered while trying to read from
+   * the socket, invoke the [onError] function. If the socket is closed by the
+   * client, invoke the [onDone] function. Only one listener is allowed per
+   * channel.
+   */
+  void listen(void onRequest(Request request),
+      {Function onError, void onDone()});
+
+  /**
+   * Send the given [notification] to the server.
+   */
+  void sendNotification(Notification notification);
+
+  /**
+   * Send the given [response] to the server.
+   */
+  void sendResponse(Response response);
+}