Fixes https://github.com/flutter/flutter/issues/70938

Co-authored-by: NSanjay Chouksey <sanjayc@google.com>

This change contains a couple of changes that should have been in
github.com/flutter/engine/commit/3105db8e
but fell through the cracks

First one lifts the initialization of the flutter::RunConfiguration so that
the asset manager gets set on the persistant cache before the shader
warmup happens. I'm not sure how this didnt end up in the first PR I
think it got mangled during merge conflict resolution. no test coverage
for that code because its in the middle of a 400 line constructor

Second one fixes a race condition that the tests dont catch because the
tests are single threaded. This change restructures the test that missed
this bug so that it would have caught that bug and will catch comparable
bugs.

* Remove null safety opt outs for dart:ui

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/fuchsia-mac-sdk-flutter-engine
Please CC jsimmons@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/master/autoroll/README.md