buf.append(t.getClass().getName());
            } else {
                buf.append(t.getMessage());
            }
            try (final StringWriter sw = new StringWriter(); final PrintWriter pw = new PrintWriter(sw)) {
                t.printStackTrace(pw);
                pw.flush();
                buf.append(" [ ").append(sw.toString()).append(" ]");

    assertWithMessage("closingExecutor was shut down")
        .that(shutdownAndAwaitTermination(closingExecutor, 10, SECONDS))
        .isTrue();
    if (!failures.isEmpty()) {
      StringWriter message = new StringWriter();
      PrintWriter writer = new PrintWriter(message);
      writer.println("Expected no failures, but found:");
      for (AssertionError failure : failures) {
        failure.printStackTrace(writer);