[flutter_tool] Only send one crash report per run

[zanderso]

Description

Prior to this PR, if there are multiple unhandled exceptions in a single invocation of the tool, they will all be sent to crash reporting. This is problematic when a stream with lots of data has a listener callback that throws a synchronous exception. The same 'crash' will generate potentially hundreds of reports until the event loop turns enough times for the tool to exit.

This PR ensures that only the first unhandled exception will be sent to crash reporting. Subsequent unhandled exceptions will still be logged to the console and/or a crash log file.

Related Issues

Noise/duplicates in crash reporting.

Tests

I added the following tests:

New test in crash_reporting_test.dart.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (Please read [Handling breaking changes]). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[jonahwilliams]

LGTM

I wonder if this explains the relatively high volume/low users of the frontend_server crash?

[jonahwilliams]

nit: I would use expect(await foo, 1)

[zanderso]

Done.

[zanderso]

I think it was the cause of the profile we are seeing from the frontend server crashers. I cycled through the error messages for a master version, and there was a discernible pattern.

[codecov]

Codecov Report

Merging #38925 into master will decrease coverage by 0.82%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master   #38925      +/-   ##
==========================================
- Coverage   56.26%   55.44%   -0.83%     
==========================================
  Files         195      195              
  Lines       18314    18316       +2     
==========================================
- Hits        10305    10155     -150     
- Misses       8009     8161     +152

Flag	Coverage Δ
#flutter_tool	55.44% <85.71%> (-0.83%)	⬇️

Impacted Files	Coverage Δ
...utter_tools/lib/src/reporting/crash_reporting.dart	76.19% <100%> (+1.19%)	⬆️
packages/flutter_tools/lib/runner.dart	71.11% <83.33%> (ø)	⬆️
...ackages/flutter_tools/lib/src/proxy_validator.dart	21.05% <0%> (-73.69%)	⬇️
packages/flutter_tools/lib/src/commands/drive.dart	5.83% <0%> (-69.17%)	⬇️
...ckages/flutter_tools/lib/src/commands/devices.dart	53.84% <0%> (-23.08%)	⬇️
...r_tools/lib/src/runner/flutter_command_runner.dart	56.19% <0%> (-13.23%)	⬇️
packages/flutter_tools/lib/src/base/context.dart	49.12% <0%> (-8.78%)	⬇️
packages/flutter_tools/lib/src/version.dart	86.89% <0%> (-5.83%)	⬇️
...ges/flutter_tools/lib/src/build_system/source.dart	81.69% <0%> (-4.23%)	⬇️
packages/flutter_tools/lib/src/device.dart	54.81% <0%> (-2.41%)	⬇️
... and 15 more

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