Add engine plugin

pkg/analyzer/lib/plugin/task.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.
+
+/**
+ * Support for client code that extends the analysis engine by adding new
+ * analysis tasks.
+ */
+library analyzer.plugin.task;
+
+import 'package:analyzer/plugin/plugin.dart';
+import 'package:analyzer/src/plugin/engine_plugin.dart';
+import 'package:analyzer/task/model.dart';
+
+/**
+ * The identifier of the extension point that allows plugins to register new
+ * analysis tasks with the analysis engine. The object used as an extension must
+ * be a [TaskDescriptor].
+ */
+final String TASK_EXTENSION_POINT_ID = Plugin.join(
+    EnginePlugin.UNIQUE_IDENTIFIER, EnginePlugin.TASK_EXTENSION_POINT);