Fix Windows dev_fs_test failure

runtime/bin/vmservice/vmservice_io.dart

Index: runtime/bin/vmservice/vmservice_io.dart
diff --git a/runtime/bin/vmservice/vmservice_io.dart b/runtime/bin/vmservice/vmservice_io.dart
index ba2b27701501b054cd2d156bc63c45886bd8d9b4..3e1bee48db0dce7c96dd69e6a0c7e7afaf032c66 100644
--- a/runtime/bin/vmservice/vmservice_io.dart
+++ b/runtime/bin/vmservice/vmservice_io.dart
@@ -86,11 +86,12 @@ Future<List<Map<String,String>>> listFilesCallback(Uri dirPath) async {
   var stream = dir.list(recursive: true);
   var result = [];
   await for (var fileEntity in stream) {
+    var filePath = new Uri.file(fileEntity.path).path;
     var stat = await fileEntity.stat();
     if (stat.type == FileSystemEntityType.FILE &&
-        fileEntity.path.startsWith(dirPathStr)) {
+        filePath.startsWith(dirPathStr)) {
       var map = {};
-      map['name'] = '/' + fileEntity.path.substring(dirPathStr.length);
+      map['name'] = '/' + filePath.substring(dirPathStr.length);
       map['size'] = stat.size;
       map['modified'] = stat.modified.millisecondsSinceEpoch;
       result.add(map);