Change hashes to use Object.create(null) instead of {}.  This eliminates all builtin properties (e.g. 'toString') on these objects that might collide with keys.

R=spertus@google.com

Review URL: https://codereview.appspot.com/15280049

git-svn-id: http://blockly.googlecode.com/svn/trunk@1442 c400ca83-b69d-9dd7-9705-49c6b8615e23