Return the element representing the method that results from looking up the given method in the superclass of this class with respect to the given library, or {@code null} if the look upfails. The behavior of this method is defined by the Dart Language Specification in section 12.15.1:
The result of looking up method m in class C with respect to library L is: - If C declares an instance method named m that is accessible to L, then that method 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 method m in S with respect to L. Otherwise, we say that the lookup has failed.
@param methodName the name of the method being looked up
@param library the library with respect to which the lookup is being performed
@return the result of looking up the given method in the superclass of this class with respectto the given library