A frontend for a standard Scala.js linker.
Produces a LinkingUnit.
You probably want to use an instance of interface.Linker, rather than
this low-level class.
Attention: a LinkerFrontend typically does not cache the IR input. It
is advisable to do so, unless all IR is already in memory.
Core specification that this linker frontend implements.
Link and optionally optimize the given IR to a LinkingUnit.