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..6a9fa6248c62c64d42005c47d01891ff9946c05e 100644
--- a/pkg/analyzer/lib/src/context/cache.dart
+++ b/pkg/analyzer/lib/src/context/cache.dart
@@ -1093,31 +1093,6 @@ enum DeltaResult {
 }
 
 /**
- * [InvalidatedResult] describes an invalidated result.
- */
-class InvalidatedResult {
-  /**
-   * The target in which the result was invalidated.
-   */
-  final CacheEntry entry;
-
-  /**
-   * The descriptor of the result which was invalidated.
-   */
-  final ResultDescriptor descriptor;
-
-  /**
-   * The value of the result which was invalidated.
-   */
-  final Object 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
  * event will be delivered first, and then delivering of the first will be