hixie: valueAsDate is defined as returning null sometimes, so it needs to be nullable. (whatwg r6862)

hixie: valueAsDate is defined as returning null sometimes, so it needs
to be nullable. (whatwg r6862)

http://dev.w3.org/cvsweb/html5/spec/Overview.html?r1=1.5461&r2=1.5462&f=h
http://html5.org/tools/web-apps-tracker?from=6861&to=6862

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RCS file: /sources/public/html5/spec/Overview.html,v
retrieving revision 1.5461
retrieving revision 1.5462
diff -u -d -r1.5461 -r1.5462
--- Overview.html	9 Dec 2011 22:01:12 -0000	1.5461
+++ Overview.html	9 Dec 2011 22:24:15 -0000	1.5462
@@ -33210,7 +33210,7 @@
            attribute DOMString <a href="#dom-input-type" title="dom-input-type">type</a>;
            attribute DOMString <a href="#dom-input-defaultvalue" title="dom-input-defaultValue">defaultValue</a>;
            attribute DOMString <a href="#dom-input-value" title="dom-input-value">value</a>;
-           attribute <span>Date</span> <a href="#dom-input-valueasdate" title="dom-input-valueAsDate">valueAsDate</a>;
+           attribute <span>Date</span>? <a href="#dom-input-valueasdate" title="dom-input-valueAsDate">valueAsDate</a>;
            attribute double <a href="#dom-input-valueasnumber" title="dom-input-valueAsNumber">valueAsNumber</a>;
            attribute unsigned long <a href="#dom-input-width" title="dom-input-width">width</a>;