[vm] `NativeCallable.isolateLocal` should enter owned isolate

[liamappelbe]

If NativeCallable.isolateLocal is invoked on a thread that is not entered into any isolate, but is the owner of the target isolate (according to the new isolate ownership API), we can enter that isolate, invoke the callback, then exit it again. Currently this case just FATALs.

Context: dart-lang/native#1908 (comment)

[biggs0125]

@liamappelbe Are you planning on taking a look at this at some point?

[liamappelbe]

Yeah, I've got a CL in development now