feat: make system config available as separate file

[jeffbcross]

I'm creating an App Shell broccoli plugin that needs to access a project's System config to properly load dependencies of an app component when pre-compiling. Right now, I would have to parse index.html to find the system configuration. Ideally, there would be a file I could import instead, i.e. system.config.json.

[jeffbcross]

This also brings up another challenge, which is that my plugin doesn't know how to locate libraries being loaded via script tags instead of by System. I would have to manually augment the System configuration in my bundle to include angular, http, router, and rxjs.

[Brocco]

Were you looking for more than just this:
index.html

<script src="system-config.js"></script>

system-config.ts

declare var System: any;

System.config({
  packages: {
    app: {
      format: 'register',
      defaultExtension: 'js'
    }
  }
});
System.import('main.js').then(null, console.error.bind(console));

[jeffbcross]

@Brocco in my case, that would work (except from the System.import included at the end, which would cause problems). It would be even better if the config could be a JSON file, so add-ons could inspect it and modify it before calling System.config

[Brocco]

FYI, I have the separation into a separate file implemented in the style guide PR #427 but would be ideal if someone could go the extra step and get that into a JSON file