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To make responsiveness to safepointing requests better when a thread does a transitive object copy we add safepoint checkins. In the fast object copy implementation - which runs inside NoSafepointScope - we bail out to the slow path if there's a safepoint requested. The slow path is safe to GC at any point due to having all objects inside handles / GC-visible objects. TEST=Existing test suite. Change-Id: Ie2c37e2f618506ab62a592aa9ff9ab266a02b64b Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/245166 Reviewed-by: Ryan Macnak <rmacnak@google.com> Commit-Queue: Martin Kustermann <kustermann@google.com>