Improves the handling of constants added in r675
(https://codereview.appspot.com/7383057).  Specifically, this removes
the dependences in the code on Maze.WALL_SQUARE being 0 and the other
square types having values greater than or equal to 1.  The hardcoding
in the definition of Maze.MAP remains for readability/writability.

Also changed some function names from like_this to likeThis.
Review URL: https://codereview.appspot.com/7424058

git-svn-id: http://blockly.googlecode.com/svn/trunk@678 c400ca83-b69d-9dd7-9705-49c6b8615e23