Test that NativeFinalizer is actually invoked.

This was originally a checkbox under dart-lang/native#669 . But I think this deserves separate issue.

Have to verify the `NativeFinalizer` is actually invoked when GC is invoked.

We should either
* Mock the finalizer in this test
* How?
* Associate an atomic counter or keep some other metric in `dartjni.so`

[https://github.com/dart-lang/ffigen/blob/master/test/native_objc_test/automated_ref_count_test.dart](this) is the test Liam mentioned in a previous discussion but I don't think this counter-decrease-on-delete pattern is applicable in Java.

* On Android, `Dart_ExecuteInternalCommand` symbol does not exist
* Is there another way to trigger GC on Android?

cc:
@liamappelbe
@dcharkes