[vm/ffi] Verify leaf calls do not do callbacks

[dcharkes]

If a leaf call decides to call back into Dart, the GC might run, which leads to errors on (or immediately after) the return sequence of the leaf call.

we should probably consider putting verification in place (under a flag enabled in profile builds?) to make catching this sort of issues easier.

Originally posted by @mraleph in #162064

[dcharkes]

Maybe we can use exit_through_ffi_. If we have a callback using NativeCallable.isolateLocal or Pointer.fromFunction, it should be an isolate local callback. So we would have had to get into the native code via an FFI call. A normal FFI call sets this field to true, a leaf call leaves it untouched.

We'd need to make sure we don't use any FFI callback in the VM / Flutter engine when having transitioned from Dart to Native as a non-FFI-call. If that's the case maybe we can make exit_through_ffi_ a bit field to avoid increasing the size of the thread object.

[mraleph]

I think leaf calls don't update Thread::execution_state_ so my idea was that we can just have an assertion at that start of a callback that it is invoked when thread is in native state. If it is generated then something is wrong. This assertion can possibly be part of TransitionNativeToGenerated. I think that it might even be lightweight enough so that we can simply enable it always (even in PRODUCT mode).

[dcharkes]

Ah right 👍 Leaf calls only update:

    __ StoreToOffset(FPREG, THR,
                     compiler::target::Thread::top_exit_frame_info_offset());
    __ StoreToOffset(branch, THR, compiler::target::Thread::vm_tag_offset());