Add AnalysisContext.onResultInvalidated(descriptor).

pkg/analyzer/lib/src/task/dart_work_manager.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.task.dart_work_manager;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/context/cache.dart';
-import 'package:analyzer/src/generated/engine.dart'
-    show
-        AnalysisEngine,
-        AnalysisErrorInfo,
-        AnalysisErrorInfoImpl,
-        AnalysisOptions,
-        CacheState,
-        InternalAnalysisContext;
+import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/utilities_collection.dart';
 import 'package:analyzer/src/task/dart.dart';
 import 'package:analyzer/src/task/html.dart';
 import 'package:analyzer/task/dart.dart';
 import 'package:analyzer/task/model.dart';
 
 /**
  * The manager for Dart specific analysis.
@@ skipping 445 matching lines @@
     }
   }
 
   bool _shouldErrorsBeComputed(Source source) =>
       context.shouldErrorsBeAnalyzed(source);
 
   static bool _isDartSource(AnalysisTarget target) {
     return target is Source && AnalysisEngine.isDartFileName(target.fullName);
   }
 }