so in the function argument list, `=None` is removed, which makes it required, so that user has to pass in something. If `None` is passed in explicitly, the Traceback is like this:
```
[songgao: /t/virgo] ./configure
Updating git submodules....
Creating GYP include files (.gypi)
Traceback (most recent call last):
  File "./configure", line 7, in <module>
    base.configure.main(None) #bundle_list_file="bundle.list")
  File "/private/tmp/virgo/base/configure.py", line 326, in main
    if not os.path.isfile(bundle_list_file):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/genericpath.py", line 29, in isfile
    st = os.stat(path)
TypeError: coercing to Unicode: need string or buffer, NoneType found
```
, which hints `bundle_list_file` needs to be a valid path to a file