Perform init-time actions based on static information in the
<polymer-element>
instance argument.
For example, the standard implementation locates the template associated
with the given <polymer-element>
and stamps it into a shadow-root to
implement shadow inheritance.
An element may override this method for custom behavior.
void parseDeclaration(Element elementElement) {
var template = fetchTemplate(elementElement);
if (template != null) {
var root = shadowFromTemplate(template);
var name = elementElement.attributes['name'];
if (name == null) return;
shadowRoots[name] = root;
}
}