v2.23 no longer works with `Promise`-based Webpack configuration

Prior to v2.23, my `webpack.config` which returns a `Promise` worked fine, but now it causes the following errors (LOTS of them):

```
Webpack config returns a `Promise`; that signature is not supported at the moment. Using empty object instead.
```

Start by locating the handling of webpack.config files that return a Promise and reproduce the reported v2.23 error. Done means Promise-based Webpack configurations are accepted without the unsupported-signature warning and the resulting configuration is processed correctly.

Written by the indexing model from the issue text.