Partly implement mischiefManaged.
In Loader.prototype.import(), build a referer and pass it to @importFor. Remove a redundant comment. Add Loader.prototype.@failLinkageUnits in anticipation of using it to handle errors after a ModuleStatus has been added to the loader.@loading map. Remove unused LinkageUnit.prototype.fail().
Remove redundant @ensureModuleExecuted call in @importFor.
Change @importFor to require a LinkageUnit and always create a LinkageUnit for a loader.import() call. Many other changes in the neighborhood.