$ copy-file Main.scala.first Main.scala
> fastLinkJS
$ copy-file target/scala-2.12/change-config-and-source-fastopt/main.js target/scala-2.12/output-1.js

# When the linker config and source both change, re-running fastLinkJS should re-link:
> set scalaJSLinkerConfig ~= (_.withOptimizer(false))
$ copy-file Main.scala.second Main.scala
> fastLinkJS
-$ must-mirror target/scala-2.12/change-config-and-source-fastopt/main.js target/scala-2.12/output-1.js
$ newer target/scala-2.12/change-config-and-source-fastopt/main.js target/scala-2.12/output-1.js
$ copy-file target/scala-2.12/change-config-and-source-fastopt/main.js target/scala-2.12/output-2.js

# However, this re-linking should not happen more than once:
> fastLinkJS
$ must-mirror target/scala-2.12/change-config-and-source-fastopt/main.js target/scala-2.12/output-2.js
-$ newer target/scala-2.12/change-config-and-source-fastopt/main.js target/scala-2.12/output-2.js
