This test isn't going to work either. The basic problem is that the valueAsDate IDL attribute in HTML5.1 specs its own type-conversion logic which (at least in my interpretation) is trumping the generic WebIDL conversion logic -- softening it from throwing exceptions.
In this case, the HTML5 spec says that anything other than a valid, non-NaN Date object will assign the value content attribute to the empty string. Then for the valueAsDate IDL attribute, on getting is interpreted as null. (Doesn't throw.)