Validate cache consistency asynchronously. Compute modification times of physical files in a separa…

pkg/analyzer/lib/file_system/memory_file_system.dart

 // Copyright (c) 2014, 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.file_system.memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:core' hide Resource;
 
@@ skipping 62 matching lines @@
     _notifyWatchers(path, ChangeType.REMOVE);
   }
 
   @override
   File getFile(String path) => new _MemoryFile(this, path);
 
   @override
   Folder getFolder(String path) => newFolder(path);
 
   @override
+  Future<List<int>> getModificationTimes(List<Source> sources) async {
+    return sources.map((source) {
+      String path = source.fullName;
+      return _pathToTimestamp[path] ?? -1;
+    }).toList();
+  }
+
+  @override
   Resource getResource(String path) {
     path = pathContext.normalize(path);
     Resource resource = _pathToResource[path];
     if (resource == null) {
       resource = new _MemoryFile(this, path);
     }
     return resource;
   }
 
   @override
@@ skipping 507 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }