Don't remove indexing operations on potential source changes.

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 memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/engine.dart' show TimestampedData;
@@ skipping 93 matching lines @@
     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.ADD);
     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('/')) {
       throw new ArgumentError("Path must start with '/'");
     }
     _MemoryResource resource = _pathToResource[path];
     if (resource == null) {
       String parentPath = posix.dirname(path);
       if (parentPath != path) {
         newFolder(parentPath);
@@ skipping 287 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }