code: function getAssociatedRadioButtons(element) {
      if (element.form) {
        return filter(element.form.elements, function(el) {
          return el != element &&
              el.tagName == 'INPUT' &&
              el.type == 'radio' &&
              el.name == element.name;
        });
      } else {
        var treeScope = getTreeScope(element);
        if (!treeScope)
          return [];
        var radios = treeScope.querySelectorAll(
            'input[type="radio"][name="' + element.name + '"]');
        return filter(radios, function(el) {
          return el != element && !el.form;
        });
      }
    }
docs: |element| is assumed to be an HTMLInputElement with |type| == 'radio'. Returns an array containing all radio buttons other than |element| that have the same |name|, either in the form that |element| belongs to or, if no form, in the document tree to which |element| belongs.  This implementation is based upon the HTML spec definition of a "radio button group": http://www.whatwg.org/specs/web-apps/current-work/multipage/number-state.html#radio-button-group