[Cleanup] fix ios warnings

[chipweinberger]

pretty self explanatory =)

this PR is based on my branch, so when the other PRs are merged it will become smaller.

the important commit is this one: https://github.com/rmawatson/flutter_isolate/pull/155/commits/e4f34020beb22bb786ce1df788cf69501a495a42

it has all been tested on macOS, iOS, android in the example app and my app