vibe.core.concurrency.1408 sometimes fails on exitEventLoop

[wilzbach]

https://ci.dlang.io/blue/organizations/jenkins/dlang-org%2Fphobos/detail/PR-6095/4/pipeline/

+ cd tests/vibe.core.concurrency.1408
+ dub --compiler=dmd --override-config=vibe-d:core/vibe-core --build-mode=separate
Performing "debug" build using dmd for x86_64.
taggedalgebraic 0.10.9: target for configuration "library" is up to date.
eventcore 0.8.27: target for configuration "epoll" is up to date.
stdx-allocator 2.77.0: target for configuration "library" is up to date.
vibe-core 1.4.0-alpha.1: target for configuration "epoll" is up to date.
vibe-d:utils 0.8.3-alpha.3: target for configuration "library" is up to date.
vibe-d:data 0.8.3-alpha.3: target for configuration "library" is up to date.
tests ~master: building configuration "application"...
../../../../.dub/packages/vibe-core-1.4.0-alpha.1/vibe-core/source/vibe/core/log.d(613,21): Deprecation: Built-in hex string literals are deprecated, use `std.conv.hexString` instead.
Linking...
To force a rebuild of up-to-date targets, run again with --force.
Running ./tests 
[2018-01-30 18:53:53.911 main(r0Px) INF] Success.
core.exception.AssertError@../../../../.dub/packages/taggedalgebraic-0.10.9/taggedalgebraic/source/taggedalgebraic.d(753): Trying to get EventSlot but have none.
----------------
??:? _d_assert_msg [0x61a55a]
Program exited with code 1
script returned exit code 2

For convenience: https://github.com/vibe-d/vibe.d/blob/master/tests/vibe.core.concurrency.1408/source/app.d