Fixes#4691Fixes#4614
This PR gives path to https://github.com/zeit/next-plugins/pull/242
I did not add or remove `^` near dependency versions in package.json files. However, I don't exclude that some changes can be made given that rc is more stable than beta.
* Examples: clarify language around Yarn create & npx
* add missing READMEs and create-next-app usage
* suggest people tag jthegedus in firebase related issues
* add yarn alt instructions
* cerebraljs example readme & fixes
* Configurable opts for babel-preset-env + babel-plugin-transform-runtime
This adds `preset-env` and `transform-runtime` options to the
`next/babel` Babel preset, which are then passed through to those
presets and transforms. This allows configuration to keep next.js
from the default 'maximum' transform, and instead use built-in
implementations of globals, classes, async, and other commonly-supported
features.
Fixes#2989
* Use spread notation instead of Object.assign
