Separate gathering Delta changes and validation.

pkg/analyzer/test/src/context/cache_test.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.test.src.context.cache_test;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/context/cache.dart';
@@ skipping 1191 matching lines @@
 /**
  * Keep the given [keepDescriptor], invalidate all the other results.
  */
 class _KeepContinueDelta implements Delta {
   final Source source;
   final ResultDescriptor keepDescriptor;
 
   _KeepContinueDelta(this.source, this.keepDescriptor);
 
   @override
+  bool gatherChanges(InternalAnalysisContext context, AnalysisTarget target,
+      ResultDescriptor descriptor, Object value) {
+    return false;
+  }
+
+  @override
+  void gatherEnd() {}
+
+  @override
   DeltaResult validate(InternalAnalysisContext context, AnalysisTarget target,
       ResultDescriptor descriptor, Object value) {
     if (descriptor == keepDescriptor) {
       return DeltaResult.KEEP_CONTINUE;
     }
     return DeltaResult.INVALIDATE;
   }
 }
 
 class _TestAnalysisTarget implements AnalysisTarget {
   @override
   Source get librarySource => null;
 
   @override
   Source get source => null;
 }