Lazy File System and Closure: length is reported as zero

[fzimmermann89]

Hi,

there seems to be another bug in libary_fs's lazy filesystem implementation:
Problem: When using closure the length of lazy loaded files is reported as zero.
Reason: Object.defineProperty(lazyArray, "length") https://github.com/kripken/emscripten/blob/incoming/src/library_fs.js#L1650

The property is created from a string name, which closure won't changed, but is later used as .length (which is changed)

Might be reasonable to fix it along with #3837 (and maybe other bugs in the lazy filesystem as they turn up during the next week..)

-Felix