Source code
You can browse the source code here:
http://code.google.com/p/base2/source/browse/#svn/trunk/src/base2/jsb
Subversion
If you want to checkout the code using SVN then you will need to checkout the entire base2 library:
http://code.google.com/p/base2/source/checkout
The jsb source is located at:
/trunk/src/base2/jsb/