Nightly Dotty workflow of 2021-07-28 failed

[github-actions]

See https://github.com/lampepfl/dotty/actions/runs/1073456889

[griggt]

It seems that one of the tests in #13130 fails only on Windows CI:

https://github.com/lampepfl/dotty/runs/3177995520?check_suite_focus=true#step:4:1163

Error:  Test dotty.tools.scaladoc.ReportingTest.testErrorInCaseOfAssetShadowing failed: scala.MatchError: List() (of class scala.collection.immutable.Nil$), took 0.296 sec
Error:      at dotty.tools.scaladoc.ReportingTest.testErrorInCaseOfAssetShadowing$$anonfun$2(RaportingTest.scala:67)
Error:      at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
Error:      at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
Error:      at dotty.tools.scaladoc.ReportingTest.checkReportedDiagnostics(RaportingTest.scala:25)
Error:      at dotty.tools.scaladoc.ReportingTest.testErrorInCaseOfAssetShadowing(RaportingTest.scala:69)
Error:      ...