Transform async functions with regenerator (#332)

* Transform async functions with regenerator

Remove `transform-async-to-generator`, which to my understanding is meant
to be used in environments that support generators natively.

Because we're compiling generators to ES5 anyway, we can simply use
`regenerator` to transform async functions too, which results in
slightly simpler output and only uses the regenerator runtime instead
regenerator runtime + _asyncToGenerator Babel helper.

* Add babel-plugin-transform-regenerator

* Use dependencies with an exact version

config/babel.dev.js

@@ -17,9 +17,10 @@ module.exports = {
   ],
   plugins: [
     require.resolve('babel-plugin-syntax-trailing-function-commas'),
-    require.resolve('babel-plugin-transform-async-to-generator'),
+    require.resolve('babel-plugin-syntax-async-functions'),
     require.resolve('babel-plugin-transform-class-properties'),
     require.resolve('babel-plugin-transform-object-rest-spread'),
+    require.resolve('babel-plugin-transform-regenerator'),
     [require.resolve('babel-plugin-transform-runtime'), {
       helpers: false,
       polyfill: false,

config/babel.prod.js

@@ -16,10 +16,11 @@ module.exports = {
   ],
   plugins: [
     require.resolve('babel-plugin-syntax-trailing-function-commas'),
-    require.resolve('babel-plugin-transform-async-to-generator'),
+    require.resolve('babel-plugin-syntax-async-functions'),
     require.resolve('babel-plugin-transform-class-properties'),
     require.resolve('babel-plugin-transform-object-rest-spread'),
     require.resolve('babel-plugin-transform-react-constant-elements'),
+    require.resolve('babel-plugin-transform-regenerator'),
     [require.resolve('babel-plugin-transform-runtime'), {
       helpers: false,
       polyfill: false,

package.json

@@ -33,11 +33,12 @@
     "babel-eslint": "6.1.2",
     "babel-jest": "14.1.0",
     "babel-loader": "6.2.4",
+    "babel-plugin-syntax-async-functions": "6.8.0",
     "babel-plugin-syntax-trailing-function-commas": "6.8.0",
-    "babel-plugin-transform-async-to-generator": "6.8.0",
     "babel-plugin-transform-class-properties": "6.11.5",
     "babel-plugin-transform-object-rest-spread": "6.8.0",
     "babel-plugin-transform-react-constant-elements": "6.9.1",
+    "babel-plugin-transform-regenerator": "6.11.4",
     "babel-plugin-transform-runtime": "6.12.0",
     "babel-preset-es2015": "6.9.0",
     "babel-preset-es2016": "6.11.3",