Add AnalysisContext.onResultInvalidated(descriptor).

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

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index def719f33a6b86f433cc2600274ea11f8b930b00..e34d1aa863c16e3ecb567d5e0bbb0cd00179e44a 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -194,6 +194,13 @@ class AnalysisContextImpl implements InternalAnalysisContext {
   ResultProvider resultProvider;
 
   /**
+   * The map of [InvalidatedResult] controllers.
+   */
+  final Map<ResultDescriptor, StreamController<InvalidatedResult>>
+      _resultInvalidatedControllers =
+      <ResultDescriptor, StreamController<InvalidatedResult>>{};
+
+  /**
    * The most recently incrementally resolved source, or `null` when it was
    * already validated, or the most recent change was not incrementally resolved.
    */
@@ -682,10 +689,14 @@ class AnalysisContextImpl implements InternalAnalysisContext {
     if (sdk == null) {
       return new AnalysisCache(<CachePartition>[_privatePartition]);
     }
-    return new AnalysisCache(<CachePartition>[
+    AnalysisCache cache = new AnalysisCache(<CachePartition>[
       AnalysisEngine.instance.partitionManager.forSdk(sdk),
       _privatePartition
     ]);
+    cache.onResultInvalidated.listen((InvalidatedResult event) {
+      _resultInvalidatedControllers[event.descriptor]?.add(event);
+    });
+    return cache;
   }
 
   /**
@@ -1104,6 +1115,13 @@ class AnalysisContextImpl implements InternalAnalysisContext {
   }
 
   @override
+  Stream<InvalidatedResult> onResultInvalidated(ResultDescriptor descriptor) {
+    return _resultInvalidatedControllers.putIfAbsent(descriptor, () {
+      return new StreamController<InvalidatedResult>.broadcast(sync: true);
+    }).stream;
+  }
+
+  @override
   CompilationUnit parseCompilationUnit(Source source) {
     if (!AnalysisEngine.isDartFileName(source.shortName)) {
       return null;