Flaky packages/flutter/test/material/date_picker_test on HHH

[mkustermann]

We have very frequent flakes of the packages/flutter/test/material/date_picker_test test. See example failure.

It does flakily reproduce by making a flutter host_debug build with HH and running this in the flutter repo:

f % cd packages/flutter
f/packages/flutter % for i in $(seq 1 10); do ../../bin/flutter test --local-engine=host_debug --local-engine-src-path=<flutter-gclient-checkout>/src test/material/date_picker_test.dart & ;done

Trying this out with src/third_party/dart@ 1e24fe7 (with the fixes from d47bb87 and 112dbab cherry-picked on top), this will fail flakily with an "EXCEPTION CAUGHT BY RENDERING LIBRARY" issue.

Running the same test on the revision before 1e24fe7, namely f005bd5, this does not seem to reproduce.

Which makes it likely that it was caused by the unoptimized monomorphic CL. If this test is not sensitive to timing changes, then it might be an actual bug.

/cc @rmacnak-google @alexmarkov