This PR fixes the following issue:
```
/C:/src/flutter/.pub-cache/hosted/pub.dartlang.org/screenshot-1.2.0/lib/screenshot.dart:45:31: Warning: Operand of null-aware operation '??' has type 'Duration' which excludes null.
 - 'Duration' is from 'dart:core'.
    return new Future.delayed(delay ?? Duration(milliseconds: 20), () async {
                              ^
```