Improved return type of PathMatcher.getMatch

Hello @nicks, 

Needed so Obvious/medium2#6272 will type-check. Also more accurate.

Please review the following commits I made in branch 'kylehardgrave-fix-getmatch-type'.

4981e70e369a6dfc93678beed53eda2dd7e05f5f (2013-06-28 12:34:42 -0700)
Improved return type of PathMatcher.getMatch


R=@nicks
VISIBLE_CHANGES=Changed type signature of PathMatcher.getMatch