styled-jsx-plugin-postcss not work with node12

[superkoh]

sample project: https://github.com/superkoh/styled-jsx-plugin-postcss-not-work-with-node12

when use node 12, the process will hang at

processor(cssWithPlaceholders, settings)
    .then(resolved)
    .catch(resolved)
loopWhile(() => wait)

seems promise can not be returned.

How to reproduce:

1. clone the project: https://github.com/superkoh/styled-jsx-plugin-postcss-not-work-with-node12
2. npm install
3. npm run dev

when use node 10, all seems ok. when use node 12, hang at the code above

[giuseppeg]

should be related to #31

[giuseppeg]

should be fixed in #35