[vm] Avoid races on Scavenger::to_.
[vm] Avoid races on Scavenger::to_.

TEST=tsan
Bug: https://github.com/dart-lang/sdk/issues/45810
Change-Id: If6a7063f66123cd38fc76db370364dbd608f93da
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/201982
Reviewed-by: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk