regression: scala/scala fails to extract after scala-parser-combinators added Scala.js support

[SethTisue]

so at e.g. https://scala-ci.typesafe.com/job/scala-2.11.x-integrate-community-build/222/console we see

[scala-parser-combinators] Cyclic reference involving 
[scala-parser-combinators]    {file:/home/jenkins/workspace/scala-2.11.x-integrate-community-build/target-0.9.5/extraction/efd45b7712336af33211e50e348aade6f7b8ba2a/projects/ee117ddc3d656a3a6fda1e525358491b8eea71fa/projects/47f78a5180eaf5c16f8b8a91d8dafb5ba30cdf3d/}scala-parser-combinatorsJS/*:crossScalaVersions
[scala-parser-combinators]    {file:/home/jenkins/workspace/scala-2.11.x-integrate-community-build/target-0.9.5/extraction/efd45b7712336af33211e50e348aade6f7b8ba2a/projects/ee117ddc3d656a3a6fda1e525358491b8eea71fa/projects/47f78a5180eaf5c16f8b8a91d8dafb5ba30cdf3d/}scala-parser-combinatorsJS/*:scalaVersion
[scala-parser-combinators]  at sbt.Dag$Cyclic.$colon$colon(Dag.scala:67)

[info] Project scala-------------------: EXTRACTION FAILED (RuntimeException: )

[SethTisue]

hmm, adding

         extra.projects: ["scala-parser-combinatorsJVM"]

changes the error to reference scala-parser-combinatorsJVM instead of scala-parser-combinatorsJS, but doesn't make it go away

[SethTisue]

dbuild does nothing involving crossScalaVersions.

it's normal in scala modules for scalaVersion to depend on crossScalaVersions; we did it that way before Sébastien's changes, and we do it in other modules too.

normally in any sbt build if you don't set crossScalaVersions explicitly it's derived from scalaVersion, so it makes sense that a cycle could arise in the course of reversing the direction of that dependency, but it's not clear to me yet why this is only happening in a dbuild context and why Sébastien's changes made it start.