threads[i] =
          new Thread() {
            @Override
            public void run() {
              for (int j = 0; j < iterations; j++) {
                Object unused = Suppliers.synchronizedSupplier(nonThreadSafe).get();
              }
            }
          };
    }
    for (Thread t : threads) {
      t.start();
    }
    for (Thread t : threads) {
      t.join();
    }

   * Returns a supplier whose {@code get()} method synchronizes on {@code delegate} before calling
   * it, making it thread-safe.
   */
  @J2ktIncompatible
  public static <T extends @Nullable Object> Supplier<T> synchronizedSupplier(
      Supplier<T> delegate) {
    return new ThreadSafeSupplier<>(delegate);
  }

  @J2ktIncompatible
  private static final class ThreadSafeSupplier<T extends @Nullable Object>

   * Returns a supplier whose {@code get()} method synchronizes on {@code delegate} before calling
   * it, making it thread-safe.
   */
  @J2ktIncompatible
  public static <T extends @Nullable Object> Supplier<T> synchronizedSupplier(
      Supplier<T> delegate) {
    return new ThreadSafeSupplier<>(delegate);
  }

  @J2ktIncompatible
  private static final class ThreadSafeSupplier<T extends @Nullable Object>