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This change makes it so that when the VM detects that a timer is at least 100 ms overdue, it sends a warning in a VM Service event on the 'Timer' stream. TEST=Built an Android Flutter app with a modified Engine that contained a `sleep` call in `eventhandler_linux.cc` and confirmed that the warning event got sent, built an iOS Flutter app with a modified Engine that contained a `sleep` call in `eventhandler_macos.cc` and verified that the warning event got sent, pkg/vm_service/test/overdue_timer_detection_test CoreLibraryReviewExempt: This CL does not include any core library API changes, only VM Service implementation changes. Change-Id: Ie8db047116b7f63cfb5413f763eaf56c7bdd6975 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/409500 Reviewed-by: Ben Konyi <bkonyi@google.com> Commit-Queue: Derek Xu <derekx@google.com>