Adds style checks to ease reviewer burden

The following checks were added in this commit:

 - mutable default args are no longer allowed
 - block comments must begin with at least one space
 - enforce assertIsNone instead of assertEqual with a None

bp more-code-style-automation

Change-Id: I7dba0c8b45078a665d40993d751c79566aa7505f
Merge "Adds style checks to ease reviewer burden"