Support flutter 1.25

[alextekartik]

Hi and thanks for your work. I was able to run your examples on the stable (1.24) flutter branch. However it does not seem to work on flutter 1.25 (currently in beta) which prevents running on the web (and on desktop on the dev branch).

Any chance to get it updated?

Thanks!

[alextekartik]

I added to RenderZBox:

  @override
  Size computeDryLayout(BoxConstraints constraints) {
    return constraints.biggest;
  }

I'm no really sure sure what I am doing but it works for my experiment on linux & web.