Migrate lib_2/developer/timeline_recorders_test.dart to null safety.

Looks like this was added to lib_2 after the initial migration.

Change-Id: Ic56357cd99c9b369c38628f4527cfa84f50f5f72
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152600
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

tests/lib/developer/timeline_recorders_test.dart

@@ -0,0 +1,23 @@
+// Copyright (c) 2020, 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.
+
+/// VMOptions=--timeline_streams=VM,Isolate,GC,Dart --timeline_recorder=endless
+/// VMOptions=--timeline_streams=VM,Isolate,GC,Dart --timeline_recorder=ring
+/// VMOptions=--timeline_streams=VM,Isolate,GC,Dart --timeline_recorder=startup
+/// VMOptions=--timeline_streams=VM,Isolate,GC,Dart --timeline_recorder=systrace
+
+import 'dart:developer';
+
+void main() {
+  Timeline.startSync('A');
+
+  Timeline.instantSync('B');
+
+  var task = new TimelineTask();
+  task.start('C');
+  task.instant('D');
+  task.finish();
+
+  Timeline.finishSync();
+}