Wait for isolate start before proceeding with expression evaluation tests

[jason-simmons]

Fixes #42359

[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[jonahwilliams]

LGTM!

[gspencergoog]

Thanks for fixing this! Can you add a Dart test for it?

[codecov]

Codecov Report

Merging #42464 into master will decrease coverage by 1.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #42464      +/-   ##
==========================================
- Coverage    60.6%   59.09%   -1.52%     
==========================================
  Files         194      194              
  Lines       18849    18849              
==========================================
- Hits        11423    11138     -285     
- Misses       7426     7711     +285

Flag	Coverage Δ
#flutter_tool	59.09% <ø> (-1.52%)	⬇️

Impacted Files	Coverage Δ
packages/flutter_tools/lib/src/desktop_device.dart	14.89% <0%> (-59.58%)	⬇️
...ges/flutter_tools/lib/src/commands/ide_config.dart	30.39% <0%> (-53.93%)	⬇️
packages/flutter_tools/lib/src/ios/simulators.dart	11.23% <0%> (-39.5%)	⬇️
packages/flutter_tools/lib/src/ios/xcodeproj.dart	75.42% <0%> (-14.29%)	⬇️
...ackages/flutter_tools/lib/src/commands/daemon.dart	21.8% <0%> (-12.07%)	⬇️
packages/flutter_tools/lib/src/base/flags.dart	38.88% <0%> (-11.12%)	⬇️
packages/flutter_tools/lib/src/device.dart	63.25% <0%> (-3.62%)	⬇️
packages/flutter_tools/lib/src/template.dart	93.65% <0%> (-1.59%)	⬇️
packages/flutter_tools/lib/src/version.dart	93.23% <0%> (-1.45%)	⬇️
...ges/flutter_tools/lib/src/application_package.dart	65.94% <0%> (-1.09%)	⬇️
... and 2 more

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