Don't remove indexing operations on potential source changes.

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 daba7f59e0151272044102f2af1baf2dc7e31547..9264f68922a663c188b425fb0206b9e5dc572b71 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -111,6 +111,17 @@ class MemoryResourceProvider implements ResourceProvider {
     return file;
   }
 
+  File updateFile(String path, String content, [int stamp]) {
+    path = posix.normalize(path);
+    newFolder(posix.dirname(path));
+    _MemoryFile file = new _MemoryFile(this, path);
+    _pathToResource[path] = file;
+    _pathToContent[path] = content;
+    _pathToTimestamp[path] = stamp != null ? stamp : nextStamp++;
+    _notifyWatchers(path, ChangeType.MODIFY);
+    return file;
+  }
+
   Folder newFolder(String path) {
     path = posix.normalize(path);
     if (!path.startsWith('/')) {