[vm, gc] Fix backwards write-barrier logic used by Object::Clone.
[vm, gc] Fix backwards write-barrier logic used by Object::Clone.

Change-Id: Ib614c0e85a96604252a47ad81ad6a8e70509a671
Reviewed-on: https://dart-review.googlesource.com/c/91242
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk