Presumably this plugin will end up with many more warnings. This will
prevent LinterPlugin.scala from getting unweildly huge. Also it removes
some boilerplate of the form
case ... if warningEnable(Warnings.ThisWarning) && ... =>
That double "Warnings.ThisWarning" was setting my teeth on edge.
Unfortunately, it introduces a pair of typecasts! I cannot convince scalac
that my references to a Global are all references to the *same* Global.