toggleAll method
Toggles all classes specified in iterable on element.
Iterate through iterable's items, and add it if it is not on it, or
remove it if it is. This is the Dart equivalent of jQuery's
toggleClass.
If shouldAdd is true, then we always add all the classes in iterable
element. If shouldAdd is false then we always remove all the classes in
iterable from the element.
Implementation
void toggleAll(Iterable<String> iterable, [bool shouldAdd]) {
iterable.forEach((e) => toggle(e, shouldAdd));
}