convert co/yield to to async/await

yield-to-async.js

// http://astexplorer.net/#/BICnPGGYdU/4

export default function ({types: t}) {
  return {
    visitor: {
      FunctionDeclaration(path) {
        if (path.node.generator) {
          path.node.async = true        
          path.node.generator = false        
        }
	  },
      YieldExpression: function(path) {
    	let argument = path.node.argument
    	path.replaceWith(t.awaitExpression(argument), false)
      }
    }
  }
}