code: private boolean isInnerSubClass(ClassSymbol c, Symbol base) {
              while (c != null && !c.isSubClass(base, types)) {
                  c = c.owner.enclClass();
              }
              return c != null;
          }
docs: Is given class a subclass of given base class, or an inner class
  of a subclass?
  Return null if no such class exists.
  @param c     The class which is the subclass or is contained in it.
  @param base  The base class