"Iterable" unexpectedly exists in ECMASCRIPT5 mode

I compile using the "--language_in ECMASCRIPT5" setting (VERBOSE mode).
But I still get an error about a type collision with an ES6 feature:
"ERROR - Variable Iterable first declared in externs.zip//es6.js". ('Iterable' is my own definition in an extern).

In ECMASCRIPT5 mode I wouldn't expect ES6 warnings or errors? Shouldn't the compiler skip the es6.js extern file in this case?

Regards
/Fredrik Blomqvist