Whitespace only mode breaks ES6 modules

[supersteves]

This is a bit like #611

It is concatenating the modules without any module scoping/loading system, and leaving the import / export statements as-is.

This means you can't debug whitespace only mode in Chrome and must wait for a longer compilation.

[Dominator008]

I'd say this is a current limitation in Chrome. IMO WHITESPACE_ONLY should keep the import / export statements as is. We can revisit this once we have a better story of how module loading works.

[rictic]

It seems like the two-argument goog.loadModule solution outlined by @jplaisted in #2647 would fix this.