Move TestOn annotation to library level (dart-lang/test_descriptor#41)
These annotations are only read at the library level. The next version
of the test package configures the `Target` for this annotation so the
analyzer will raise diagnostics for these incorrect usages.
diff --git a/pkgs/test_descriptor/test/file_test.dart b/pkgs/test_descriptor/test/file_test.dart
index 8b111f9..13b7597 100644
--- a/pkgs/test_descriptor/test/file_test.dart
+++ b/pkgs/test_descriptor/test/file_test.dart
@@ -2,8 +2,9 @@
// 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.
-import 'dart:convert';
@TestOn('vm')
+
+import 'dart:convert';
import 'dart:io';
import 'package:path/path.dart' as p;
