Return the element representing the method that results from looking up
methodName 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 16.15.1:
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.
MethodElement lookUpMethod(String methodName, LibraryElement library);