public void onSuccess(String result) {
      synchronized (monitor) {
        assertFalse(wasCalled);
        wasCalled = true;
        assertEquals(value, result);
      }
    }

    @Override
    public synchronized void onFailure(Throwable t) {
      synchronized (monitor) {
        assertFalse(wasCalled);
        wasCalled = true;
        assertEquals(failure, t);
      }
    }
  }