Can't replace components by adding sources

When you invoke `container.load()`, it pushes the new source onto the end of an array of sources. What I am trying to do is the opposite -- I want to take my application's existing container, and when setting up the test suites, add a source whose components will shadow the existing ones. Right now, I can do this with `_registerModule()`, but that is obviously private API and not to mention quite clunky.

Is there a reason that you push to sources instead of unshifting? I would normally expect those added later to take precedence, but I'm not sure if there are situations I haven't yet considered. Perhaps there is also a better solution that I haven't considered yet.