[vm/ffi] Expose persistent handles to Dart through FFI

[sjindel-google]

The native API (dart_api.h) exposes persistent handles to native code through pointers. We should add support to the FFI to creating persistent handles in dart (as ffi.Pointers) which can be passed back into native calls. This would be useful, for example, for passing state through native callbacks.

I imagine the API would look something like:

abstract class PersistentHandle extends Pointer<Void> {
  PersistentHandle(Object target) native "...";
  Object get target native "...";
  set target(Object val) native "...";
  void destroy() native "...";
}

[sjindel-google]

This would unlock asynchronous callbacks by enabling Dart to pass SendPorts into C/C++ directly.