According to js-loaders, a Loader() ctor takes an object argument; I call ours an instance of LoaderHooks. All of the calls -- except for one -- go from Loader to LoaderHooks. The one exception is our call from getModuleDescriptionForSpecifier() to loader.getCodeUnit(name). So I think there is a problem in the design, but I don't have a solution yet. In the meantime somehow we need to get loader or the lookup call into getModuleDescriptionForSpecifier(). 

Here I am setting the `this.loader` so we can call `this.loader.getCodeUnit()` in getModuleDescriptionForSpecifier(). It's lame.