Normalize all RequestWrapper lifecycle methods to return a Promise<*>

[jeffposnick]

Library Affected:
workbox-runtime-caching

The method signature for the public cacheWillMatch() interface is for it to return a Response, but there are use cases in which it would be ideal to perform some asynchronous operation prior to returning that response. In the next major release, we should change the interface to return a Promise<Response> instead.

The concrete use case for this is to perform an IndexedDB lookup prior to returning a response, which would have come in handy when fixing #691

[addyosmani]

In the next major release, we should change the interface to return a Promise instead.

I was about to ask about the use-case, but you have that covered :) Making this change in the next major release SGTM.

[gauntface]

Should we make this consistent across all plugins, i.e. they all return a Promise?

[jeffposnick]

That would probably future proof everything, yes.

[addyosmani]

Should we make this consistent across all plugins, i.e. they all return a Promise?

👍 Agree with going down this route for consistency sake. Do we need a separate issue for tracking doing this or did you want to reuse the same issue for it?

[gauntface]

Reuse this one.

[jeffposnick]

This was fixed in #736