Add AnalysisContext.onResultInvalidated(descriptor).

pkg/analyzer/lib/src/context/cache.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.
 
 library analyzer.src.context.cache;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/src/dart/element/element.dart'
@@ skipping 1077 matching lines @@
 
   /**
    * Keep this result and stop visiting results that depend on this one.
    */
   STOP
 }
 
 /**
  * [InvalidatedResult] describes an invalidated result.
  */
-class InvalidatedResult {
+class InvalidatedResult<V> {
   /**
    * The target in which the result was invalidated.
    */
   final CacheEntry entry;
 
   /**
    * The descriptor of the result which was invalidated.
    */
-  final ResultDescriptor descriptor;
+  final ResultDescriptor<V> descriptor;
 
   /**
-   * The value of the result which was invalidated.
+   * The value of the result before it was invalidated, may be the default
+   * value if the result was flushed.
    */
-  final Object value;
+  final V value;
 
   InvalidatedResult(this.entry, this.descriptor, this.value);
 
   @override
   String toString() => '$descriptor of ${entry.target}';
 }
 
 /**
  * A Stream-like interface, which broadcasts events synchronously.
  * If a second event is fired while delivering a first event, then the second
@@ skipping 145 matching lines @@
   void resultAccessed(TargetedResult result) {}
 
   @override
   List<TargetedResult> resultStored(TargetedResult newResult, newValue) {
     return TargetedResult.EMPTY_LIST;
   }
 
   @override
   void targetRemoved(AnalysisTarget target) {}
 }