Return the element representing the setter that results from looking up
setterName in this class with respect to the given
null if the look up fails. The behavior of this method is defined by
the Dart Language Specification in section 16.15.2:
The result of looking up getter (respectively setter) m in class C with respect to library L is: If C declares an instance getter (respectively setter) named m that is accessible to L, then that getter (respectively setter) is the result of the lookup. Otherwise, if C has a superclass S, then the result of the lookup is the result of looking up getter (respectively setter) m in S with respect to L. Otherwise, we say that the lookup has failed.
PropertyAccessorElement lookUpSetter( String setterName, LibraryElement library);