Include `babel-polyfill` or use `transform-runtime`

[AndersDJohnson]

Should include babel-polyfill as a runtime dependency, since it's imported by the es6 source, and the transpiled code ends up with require statements for core-js and regenerator-runtime. See http://babeljs.io/docs/usage/polyfill/#installation.

Though generally it's preferred to use the transform runtime plugin for libraries, since that won't pollute the global scope.