Add AnalysisContext.onResultInvalidated(descriptor).

pkg/analyzer/lib/src/context/cache.dart

Index: pkg/analyzer/lib/src/context/cache.dart
diff --git a/pkg/analyzer/lib/src/context/cache.dart b/pkg/analyzer/lib/src/context/cache.dart
index bbb0e698e3acb1add0ae5e9e3952a3eab1b12b99..de913f3c72bc204d6060ce542dfce7c43f7e98b4 100644
--- a/pkg/analyzer/lib/src/context/cache.dart
+++ b/pkg/analyzer/lib/src/context/cache.dart
@@ -1095,7 +1095,7 @@ enum DeltaResult {
 /**
  * [InvalidatedResult] describes an invalidated result.
  */
-class InvalidatedResult {
+class InvalidatedResult<V> {
   /**
    * The target in which the result was invalidated.
    */
@@ -1104,12 +1104,13 @@ class InvalidatedResult {
   /**
    * 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);