85b79f0856
This is prep work for allowing the main thread to contribute during idle time or when waiting for the sweeper to catch up. TEST=ci Change-Id: Ia25c61a448bf5d2dada7d5b494ea08f3526d8740 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/228725 Reviewed-by: Siva Annamalai <asiva@google.com> Commit-Queue: Ryan Macnak <rmacnak@google.com>