Router: Enable loadChildren to support a function that returns a Promise #9938

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[ ] bug report
[x] feature request
[ ] support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Current behavior

The loadChildren route config property only accepts a string

Expected/desired behavior

The loadChildren route config property accepts a string or function that returns a promise

Reproduction of the problem
If the current behavior is a bug or you can illustrate your feature request better with an example, please provide the steps to reproduce and if possible a minimal demo of the problem via https://plnkr.co or similar (you can use this template as a starting point: http://plnkr.co/edit/tpl:AvJOMERrnz94ekVua0u5).

What is the expected behavior?

What is the motivation / use case for changing the behavior?

In order to allow lazy loading in Webpack 1 with the upcoming @AppModules, there needs to be a way to return a promise that resolves an app module.

const routes: Routes = [
  {
    path: 'parent',
    loadChildren: () => new Promise(resolve => {
      require.ensure([], require => {
        resolve(require('./parent/module').ParentAppModule);
      })
    })
  }
];

It would also be helpful if this support were added to the SystemJsAppModuleLoader so SystemJS and Webpack are supported out of the box.

Please tell us about your environment:

• Angular version: 2.0.0-rc.X
• Browser: [all | Chrome XX | Firefox XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]
• Language: [all | TypeScript X.X | ES6/7 | ES5 | Dart]

cc: @vsavkin @tbosch