code: public MethodSymbol firstUnimplementedAbstract(ClassSymbol sym) {
          try {
              return firstUnimplementedAbstractImpl(sym, sym);
          } catch (CompletionFailure ex) {
              chk.completionError(enter.getEnv(sym).tree.pos(), ex);
              return null;
          }
      }
docs: Return first abstract member of class `sym'.