code: @SuppressForbidden("legitimate sysstreams.")
    public static void warn(String message, Throwable t) {
      PrintStream w = (warnings == null ? System.err : warnings);
      try {
        w.print("WARN: ");
        w.print(message);
        if (t != null) {
          w.print(" -> ");
          try {
            t.printStackTrace(w);
          } catch (OutOfMemoryError e) {
            // Ignore, OOM.
            w.print(t.getClass().getName());
            w.print(": ");
            w.print(t.getMessage());
            w.println(" (stack unavailable; OOM)");
          }
        } else {
          w.println();
        }
        w.flush();
      } catch (OutOfMemoryError t2) {
        w.println("ERROR: Couldn't even serialize a warning (out of memory).");
      } catch (Throwable t2) {
        // Can't do anything, really. Probably an OOM?
        w.println("ERROR: Couldn't even serialize a warning.");
      }
    }
docs: Warning emitter. Uses whatever alternative non-event communication channel is.