Babel Transform JSX to React.jsx/React.jsxDEV Plugin (#16432)

This babel transform is a fork of the @babel/plugin-transform-react-jsx transform and is for experimentation purposes only. We don't plan to own this code in the future, and we will upstream this to Babel at some point once we've proven out the concept.

As per the RFC to simplify element creation, we want to change the JSX transform from targeting React.createElement(type, props, children) to React.jsx(type, props, key). This modifies the existing @babel/plugin-transform-react-jsx (and helper) babel plugin to support React.jsx and React.jsxDEV.

The main differences between React.jsx/React.jsxDEV and React.createElement are:
1.) key is now passed as an explicit argument rather than through props
3.) children are now passed through props rather than as an explicit argument
4.) props must always be an object
5.) __source and and __self are now passed as separate arguments into React.jsxDEV rather than through props

Part of the rationale for this change is that we want to deprecate key spread through props because this is an expensive dynamic comparison operation. We want users instead always explicitly pass key as a prop. However, in the interim, we need a way to distinguish between <div {...props} key={foo} /> and <div key={foo} {...props} />. Therefore, until we completely deprecate key spreading, we will use React.createElement to transform <div {...props} key="Hi" /> and React.jsx to transform everything else.

package.json

@@ -4,6 +4,7 @@
     "packages/*"
   ],
   "devDependencies": {
+    "@babel/plugin-syntax-jsx": "^7.2.0",
     "@babel/cli": "^7.0.0",
     "@babel/code-frame": "^7.0.0",
     "@babel/core": "^7.0.0",

packages/babel-plugin-react-jsx/README.md

@@ -0,0 +1,5 @@
+This package is intended to eventually replace the current `@babel/plugin-transform-react-jsx`, changing the JSX transform from targeting `React.createElement(type, props, children)` to `React.jsx(types, props, key)`. 
+
+https://github.com/reactjs/rfcs/blob/createlement-rfc/text/0000-create-element-changes.md
+
+**This is experimental and not intended to be used directly.**