Add top-level driver and abstract plugin superclass

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

Index: pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart
diff --git a/pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart b/pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart
index 7472483872f7acae0a48e0bbe58ac7944c5eb653..a7ccafc82ea823b79288bdc92ce7d67ccff7953f 100644
--- a/pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart
+++ b/pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart
@@ -10,7 +10,8 @@ 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 {
   /**