* Mark transpiled JSX elements as pure
* Avoid duble annotation
* Add "pure" option to the React preset
* Fix generator indentation
* Update tests
* Add tests for the "pure" option
* Update windows fixtures
While it isn't technically needed because regenerator already handles
async functions, it doesn't play well with the Promise detection logic
used by @babel/preset-env's useBuiltIns and @babel/transform-runtime.
The plugin exclusion logic isn't removed, because
1) it will be useful when merging @babel/preset-modules
into @babel/preset-env
2) if/when regenerator will support injecting a custom promise,
we can enable this optimization again
