This resolves #2349. It fixes two bugs:

1. When loading a bundle via System.import() with named registers, it is no longer possible for the first named register in the bundle to instantiate twice. Previously this was true, as the register array was instantiated once for the bundle of the module and once for the name it gave itself via a named register.
2. The named-register extra now always returns the first named register, rather than sometimes the first named define and sometimes the last named register

Demonstration of the bugs:

1. (todo)
2. Last named register returned (script-load.js) and First named register returned (transform.js)

The changes I had to make to solve these bugs were more invasive than I was hoping for. It might be safest to release this is a new major version, as people are probably accidentally relying on the named-register extra to sometimes return the first named register and other times the last named register.

I'm open to all feedback on this one, as it was quite difficult and I tried several approaches before finding one that worked in all cases. The first bug is solved by creating a new namedRegisterAliases property on systemjs instances, to ensure that only one module is instantiated. The second bug is solved by switching from Promise.resolve to setTimeout, since the load event for <script> events seems to run after the Promise.resolve() executes.

Adding the url to the getRegister API was something I wanted to avoid, but ultimately was the least invasive approach I found that worked.

@guybedford I would appreciate your review on this one, if you're able to.