[ffi] Fix memory leaks

[dcharkes]

Closes #93.

Native memory allocated in Dart is freed in Dart (.toNativeUtf8() -> calloc.free()).

Native memory allocated in C is freed in C (malloc -> free).

This symmetry makes it such that we never end up calling free on something malloced from another definition of malloc than the one that is in the same native library as free.

[nmfisher]

I've written some code that mallocs a char pointer on the C side, passes back to Dart via FFI then calls toDartString() and calloc.free.

Judging from this commit, though, my code should actually be crashing but it's worked fine so far (admittedly it's basic demo code).

Any clarification? Is toDartString() calling free on the C-side under the hood somewhere?

[dcharkes]

I've written some code that mallocs a char pointer on the C side, passes back to Dart via FFI then calls toDartString() and calloc.free.

This will most likely work fine as long as you're not on Windows. And on Windows it can work, but not always.

See https://stackoverflow.com/a/8722008/8136619

[nmfisher]

Thanks!