function getElementStyleSheet(element) {
  assertHTMLElement(element);
  return genericPropertyGet(
      Methods.SHEET_GETTER, element, 'sheet', function(sheet) {
        return sheet instanceof CSSStyleSheet;
      });
}
Returns an element's style without falling prey to things like
<form><input name="style"></form>.
@param {!Element} element
@return {!CSSStyleSheet}