DDC Firefox testing has flaky async test errors

[whesse]

We are seeing continual errors on DDK Firefox in the language async tests, that are unrelated to the commits they appear on.
These are making it past the deflaking, which seems to indicate their failures are grouped somehow.

We need to solve this, because it is causing failure reports on the result feed that people need to ignore and/or approve.

Here are some tests that fail flakily, all with RuntimeError:

language_2/async_star/async_star_await_for_test
language_2/async_star/async_star_test
language_2/deferred/redirecting_factory_test
language_2/async_star/async_star2_test/02
language_2/async_star/cancel_while_paused_test

Comment:
https://dart-ci.appspot.com/log/ddk-linux-release-firefox/dartdevk-checked-linux-release-firefox/1432/language_2/async_star/cancel_while_paused_test :

-- Command "firefox" (took 02.000933s):
/b/s/w/ir/cache/builder/sdk/tools/sdks/dart-sdk/bin/dart pkg/test_runner/bin/launch_browser.dart firefox http://127.0.0.1:43437/root_build/generated_tests/dartdevk-firefox-checked-sdk/tests_language_2_async_star_cancel_while_paused_test/test.html?crossOriginPort=36455

Runtime window.onerror:
window.onunhandledrejection called:

DartError@http://127.0.0.1:43437/root_build/gen/utils/dartdevc/kernel/amd/dart_sdk.js:5166:7
throw_@http://127.0.0.1:43437/root_build/gen/utils/dartdevc/kernel/amd/dart_sdk.js:3875:11
_fail@http://127.0.0.1:43437/root_build/gen/utils/dartdevc/pkg_kernel/async_helper.js:620:12
listEquals@http://127.0.0.1:43437/root_build/gen/utils/dartdevc/pkg_kernel/async_helper.js:424:23
main/</<@http://127.0.0.1:43437/root_dart/out/ReleaseX64/generated_compilations/dartdevk-checked-sdk/tests_language_2_async_star_cancel_while_paused_test/cancel_while_paused_test.js:76:23
delayed/<@http://127.0.0.1:43437/root_build/gen/utils/dartdevc/kernel/amd/dart_sdk.js:27371:33
internalCallback@http://127.0.0.1:43437/root_build/gen/utils/dartdevc/kernel/amd/dart_sdk.js:18653:11