add file tests; upgrade chrome_gen version to capture a file access bug ...

There was a regression in getting the contents of a file, due to the latest dart:js changes. This CL:

* adds tests for basic file operations, so we know when something breaks
* revs our dependency on package:chrome_gen, to capture the bug fix to readText()

The 'open file' button now populates the editor w/ the file contents again.

@gaurave