Fixed issue #556: nonopening blocks drawers in Safari on MacOSX.

The problem was my (= Lyn's) failure to appropriately abstract over
the code for getting the children of an HTML DOM element. For this purpose,
do *not* use <element>.children or <element>.childNodes; instead use
goog.dom.getChildren(<element>).

Change-Id: I18205cce71fdd1143d10afb97382b1dd305f6c4c