feat(platform-browser): allow lazy-loading HammerJS

[jelbourn]

Open for discussion. I made the loader Promise-based instead of Observable for simplicity here, but can change it if folks feel strongly.

[mary-poppins]

You can preview 950f46f at https://pr23906-950f46f.ngbuilds.io/.

[Toxicable]

This might be a reasonable case for import()?

[jelbourn]

@Toxicable the goal is to be agnostic to any particular loading implementation

[alfaproject]

I’ve been telling people to stop using Promises because Observables are more powerful. I think it would be good to be consistent in the framework side too

[vicb]

Is | null correct here ?

[jelbourn]

I believe so; null is an acceptable value to provide for the token

[vicb]

I think | null should be here instead ?

[jelbourn]

Per other comment: null is an acceptable value to provide for the token

[mhevery]

You need to add public docs and @stablehere

[petebacondarwin]

@stable is no longer needed nor permitted. It is implied from a lack of @deprecated and @experimental tags.

[cyrilletuzi]

@jelbourn Is there documentation somewhere to know how to use this feature? Thanks.

[jelbourn]

No documentation yet, but it boils down to providing a function as HAMMER_LOADER, where the function returns a Promise that is resolved once Hammer is available on the page. The function can load Hammer however you want.