Add top-level driver and abstract plugin superclass

pkg/analyzer_plugin/lib/src/driver.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:isolate';
+
+import 'package:analyzer_plugin/plugin/plugin.dart';
+import 'package:analyzer_plugin/src/channel/isolate_channel.dart';
+import 'package:analyzer_plugin/starter.dart';
+
+/**
+ * The [Driver] class represents a single running instance of an analysis
+ * server plugin. It is responsible for handling the communications with the
+ * server and forwarding requests on to the plugin.
+ */
+class Driver implements ServerPluginStarter {
+  /**
+   * The plugin that will be started.
+   */
+  final ServerPlugin plugin;
+
+  /**
+   * Initialize a newly created driver that can be used to start the given
+   * plugin.
+   */
+  Driver(this.plugin);
+
+  @override
+  void start(SendPort sendPort) {
+    IsolateChannel channel = new IsolateChannel(sendPort);
+    plugin.start(channel);
+  }
+}