Support es6 exports from --entry_point

Support export statements from the entry point file. This will enable many requested features such as

import chunking (e.g., Dynamic Import Support)
dynamic import

while giving the user precise control.

Given a file

// entry.js
import { m1, m2 } from "./mod"
...
...
export { sym1, ..., symN }
export default symDefault;

When compiled with

google-closure-compiler -O ADVANCED --js mod.js entry.js --entry_point entry.js --module_resolution NODE

should output a file containing export { sym1, ..., symN, symDefault as default}.

Currently, this behavior can be achieved by assigning each exported symbol to globalThis in a preprocessing step and then replacing each such assignment in the compiled file to an export statement in a postprocessing step.

google / closure-compiler

Support es6 exports from --entry_point #4167