code: public void warning(LintCategory lc, Warning warningKey) {
          report(diags.warning(lc, null, null, warningKey));
      }
docs: Report a lint warning, unless suppressed by the  -nowarn option or the
  maximum number of warnings has been reached.
  @param lc     The lint category for the diagnostic
  @param warningKey    The key for the localized warning message.