Investigate the persistent failure in the flutter CI bot

[devoncarew]

See here for an example failure:

https://travis-ci.org/github/dart-lang/dartdoc/builds/718426662

...
flutter-docs: Precompiled grinder:_boot.
flutter-docs: Precompiled grinder:_boot.
flutter-docs: Precompiled grinder:_boot.
flutter-docs: Precompiled grinder:grinder.
flutter-docs: Precompiled grinder:init.
flutter-docs: Precompiled coverage:format_coverage.
flutter-docs: Precompiled coverage:format_coverage.
flutter-docs: Precompiled coverage:run_and_collect.
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#build-times-out-because-no-output-was-received
The build has been terminated

[devoncarew]

Possibly a dup of #2233 ?

[jcollins-g]

looking at this and similar hangs in the CI I think it is a deadlock in MultiFutureTracker, probably due to a change in how Future.next() is executed. It's extremely hard to tell just by inspection though -- I'll poke at it a bit.

[jcollins-g]

from Future.then: "If this future is already completed, the callback will not be called immediately, but will be scheduled in a later microtask.". MultiFutureTracker depends on the callback being called immediately after the completion of the Future in order not to deadlock. PR coming up.

EDIT: This is wrong. The deadlock was actually due to missing exception handling. If callbacks are not called immediately after the completion of the future it's possible for this to not be efficient or livelock (we may wind up waiting on the same Future multiple times depending on microtask scheduling), but it shouldn't deadlock. This turned out to be hard to test for because depending on how you create Futures in a test it will change how the microtasks are executed.

[jcollins-g]

I'm still working on the deadlock and it isn't the only problem. We are also impacted by dart-lang/sdk#43487 as our coverage runs require passing special flags to the Dart VM.