Flaky FlutterChannelsTest tests

[flar]

I'm seeing rollers fail on the Linux Mac Unopt task with various failures in the FlutterChannelsTest. For example: https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20mac_unopt/946/overview

The run with the most failures included the following failures, but other runs on the same bot (it does 3 retries) had only a subset of these failures:

Failing tests:
	-[FlutterChannelsTest testBasicMessageChannelInvokeHandlerAfterChannelReleased]
	-[FlutterChannelsTest testBasicMessageChannelTaskQueue]
	-[FlutterChannelsTest testEventChannelTaskQueue]
	-[FlutterChannelsTest testMethodChannelInvokeHandlerAfterChannelReleased]
	-[FlutterChannelsTest testMethodChannelTaskQueue]

[flar]

See also #126013 reported by @dnfield

[flar]

Looking through the logs for "successful" runs of this task, it looks like these tests are usually run more than once specifically due to failures on these tests, so this is a sever flake that is wasting automated testing resources. See https://ci.chromium.org/p/flutter/builders/try/Linux%20mac_unopt

[dnfield]

This is a duplicate of #126013

[jmagman]

@flar I merged flutter/engine#41747, please reopen #126013 if you see this flaking again.

[github-actions]

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