Adding alt text to image fields for use when block collapses.  Previously the image URL was used.

R=spertus@google.com

Review URL: https://codereview.appspot.com/50510043

git-svn-id: http://blockly.googlecode.com/svn/trunk@1605 c400ca83-b69d-9dd7-9705-49c6b8615e23
Revert the bulk of r674.  Dart is back\!

git-svn-id: http://blockly.googlecode.com/svn/trunk@1606 c400ca83-b69d-9dd7-9705-49c6b8615e23