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

pkg/analyzer/lib/file_system/memory_file_system.dart

Index: pkg/analyzer/lib/file_system/memory_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/memory_file_system.dart b/pkg/analyzer/lib/file_system/memory_file_system.dart
index 3391433d314173a4f1e37ae585fc34b3e07a23ea..1943ddf2ba4f21535b3d230dea190912c357ec2f 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -80,6 +80,14 @@ class MemoryResourceProvider implements ResourceProvider {
   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];