Revert "Change the way tasks are registered" (TBD)

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;
 
@@ skipping 23 matching lines @@
 /**
  * The identifier of the extension point that allows plugins to register new
  * analysis error results to compute for an HTML source. The object used as an
  * extension must be a [ResultDescriptor].
  */
 final String HTML_ERRORS_EXTENSION_POINT_ID = Plugin.join(
     EnginePlugin.UNIQUE_IDENTIFIER, EnginePlugin.HTML_ERRORS_EXTENSION_POINT);
 
 /**
  * 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].
+ *
+ * Contributed tasks should never extract information from Dart elements or AST
+ * (e.g. offsets) of a source and put it into results for targets in other
+ * sources. Dart elements and ASTs are updated during incremental resolution,
+ * and invalidation of results is intentionally limited by the source bounds
+ * for performance reasons.
+ */
+final String TASK_EXTENSION_POINT_ID = Plugin.join(
+    EnginePlugin.UNIQUE_IDENTIFIER, EnginePlugin.TASK_EXTENSION_POINT);
+
+/**
+ * The identifier of the extension point that allows plugins to register new
  * work managers with the analysis engine. The object used as an extension must
  * be a [WorkManagerFactory].
  */
 final String WORK_MANAGER_EXTENSION_POINT_ID = Plugin.join(
     EnginePlugin.UNIQUE_IDENTIFIER,
     EnginePlugin.WORK_MANAGER_FACTORY_EXTENSION_POINT);
 
 /**
  * A function that will create a new [WorkManager] for the given [context].
  */
 typedef WorkManager WorkManagerFactory(InternalAnalysisContext context);