`iso-stress-linux` failed due to unexpected "concurrent modification during iteration" error

[derekxu16]

[/b/s/w/ir/cache/builder/sdk/runtime/tests/concurrency/../../../tests/language/await/await_test.dart] error: [Concurrent modification during iteration: _List len:9., #0      ListIterator.moveNext (dart:_internal/iterable.dart:365)
#1      controlFlow (file:///b/s/w/ir/cache/builder/sdk/tests/language/await/await_test.dart:140)
<asynchronous suspension>
#2      asyncSuccess (package:expect/async_helper.dart:90)
<asynchronous suspension>
]
------------------------------------------------------------
-> Setting exitCode to 255
INFO: No unexpected crashes recorded

https://ci.chromium.org/ui/p/dart/builders/ci.sandbox/iso-stress-linux/7533/overview

[rmacnak-google]

This has the same cause as #60017.