The only officially available "dev build" is nightly.flarum.site which is part of our demo forums https://discuss.flarum.org/d/21101-demos-come-to-flarum
But right now most of our work (Mithril v2) is happening in PRs that haven't been merged yet so those changes cannot be previewed on the nightly website at this time. As askvortsov says, the only way to test that code would be to manually download core and extension branches/commit just like you would work on an extension.
It's important to note that the javascript is not compiled on those branches/PRs, so you would also need to compile it yourself after the checkout.
