Return promise from component.preload()

[parties]

Preface: I'm not sure if this is the "correct" solution to my problem (maybe I missed something in the documentation) but this was the only way I could find to hook into the process that actually loads the component.

Summary

The problem I've been experiencing is that I needed to be able to call Component.preload() to load a component, but also wait for preload() to complete before rendering the route.

(We're stuck using react-router 3.x for the time being...)

This PR solves our problem by simply returning ctor. requireAsync(props) from the preload() function, as requireAsync already returns a promise that resolves once the component is loaded.

Test plan

Where are the tests for loadable/component? Am I missing something?

[parties]

Closing because I'm bad at searching:

• Make loadable.preload() return a promise #285
• Loadable.preload return the promise #226