code: public void mandatoryWarning(LintCategory lc, DiagnosticPosition pos, Warning warningKey) {
          report(diags.mandatoryWarning(lc, source, pos, warningKey));
      }
docs: Report a warning.
  @param lc     The lint category for the diagnostic
  @param pos    The source position at which to report the warning.
  @param warningKey    The key for the localized warning message.