I was assuming that, in the in-browser branch, we'd change the require.config to redefine `"fileSystemImpl"` to point to a different impl module.  Otherwise I'm confused about why we need that level of indirection: if we're just changing the contents of the same fixed module, we could load it the regular way with no special require.config needed.

If we're keeping the require.config indirection, then I think renaming to something like InNativeShell or InAppShell would be clearer -- since we'd have a different file in the in-browser branch.