System.import()?

[Jason Orendorff]

The ES6 module system is taking a real beating in the comments section
here: https://hacks.mozilla.org/2015/08/es6-in-depth-modules/

People are concerned about things like:

- There is no standard way to load any modules at all in the browser.
- There is no standard way for a module to load other modules later
(lazily, for faster initial load times).
- There is no standard way to conditionally load modules.
- There is no standard way to catch errors when module loading fails.

There's a planned feature that addresses all these use cases:
`System.import(moduleSpec, referrer)`.

It's possible to make minor changes to HostResolveImportedModule and
then specify `System.import` in terms of that. It could ship in the
existing compilation-plus-polyfill module system implementations (like
webpack) immediately. And it'd be fully compatible with the coming JS
Loader Standard.

Arguably something this fundamental to module usage belongs in ECMA-262 anyway.

What do you think?

-j