Renamed default exports generates invalid ES6 module code

[Victorystick]

Rollup sometimes generates invalid code when targeting es6 modules.

This module

// main.js
import foo from './bar.js';
export default foo;

importing that

// bar.js
var bar = 1;
export default bar;

generates

// output.es6.js
var bar = 1;
export default foo;

None of the other formats have this bug. However, this other example which does basically the same thing breaks all formats except es6 with TypeError: Cannot read property 'render' of undefined.

// main.js
export { default as default } from './bar.js';