Merge this repo into dart-lang/native

[dcharkes]

As discussed on #430, we'd like to merge this repo with dart-lang/native.

I don't believe we have to follow the history rewrite steps in dart-lang/tools#100, because this repo is already a mono-repo.

We do need the other steps from that though.

And at least one additional step:

• toplevel readme should be moved into package:jnigen

@HosseinYousefi could you prepare a PR with the steps linked above?