Previously the synthetic field that holds the FfiNative
function pointer was injected into the current library.
This change makes sure we instead add the field to the
relevant parent - Class or Library.
TEST=Added regression test for name collision.
Bug: https://github.com/dart-lang/sdk/issues/43889
Change-Id: Ifbf2d70de00e4748c179fe7d626c495675c2b338
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/208502
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Clement Skau <cskau@google.com>
