Get memento objects from tasks, remember in cache and pass back to tasks.

pkg/analyzer/lib/task/model.dart

Index: pkg/analyzer/lib/task/model.dart
diff --git a/pkg/analyzer/lib/task/model.dart b/pkg/analyzer/lib/task/model.dart
index 973f24ee91dc3005cd90612e3e1566b44205f29d..ccd87b7d1c8fa18a2628c215da8ae1945fdec35c 100644
--- a/pkg/analyzer/lib/task/model.dart
+++ b/pkg/analyzer/lib/task/model.dart
@@ -103,6 +103,12 @@ abstract class AnalysisTask {
   Map<String, dynamic> inputs;
 
   /**
+   * The optional data that the task associated with [target] last time.
+   * This data may help to compute outputs more efficiently.
+   */
+  dynamic inputMemento;
+
+  /**
    * A table mapping result descriptors whose values are produced by this task
    * to the values that were produced.
    */
@@ -110,6 +116,12 @@ abstract class AnalysisTask {
       new HashMap<ResultDescriptor, dynamic>();
 
   /**
+   * An optional data that the task wants to associate with [target].
+   * This data may help later to compute outputs more efficiently.
+   */
+  dynamic outputMemento;
+
+  /**
    * The exception that was thrown while performing this task, or `null` if the
    * task completed successfully.
    */
@@ -389,7 +401,7 @@ abstract class TaskDescriptor {
    * used to compute results based on the given [inputs].
    */
   AnalysisTask createTask(AnalysisContext context, AnalysisTarget target,
-      Map<String, dynamic> inputs);
+      Map<String, dynamic> inputs, dynamic inputMemento);
 }
 
 /**