Add the class value to element.
add and addAll are the Dart equivalent of jQuery's addClass.
If this CssClassSet corresponds to one element. Returns true if value was
added to the set, otherwise false.
If this corresponds to many elements, null is always returned.
value must be a valid 'token' representing a single class, i.e. a
non-empty string containing no whitespace. To add multiple classes use
addAll.
Source
bool add(String value);