testDelete() {
    const m = new StructsMap();
    for (let i = 0; i < 1000; i++) {
      m.set(i, 'foo');
    }

    for (let i = 0; i < 1000; i++) {
      assertTrue(m.keys_.length <= 2 * m.size);
      m.delete(i);
    }
    assertEquals(0, m.size);
    assertEquals('', Array.from(m.keys()).join(''));
  }
@suppress {visibility} checking private keys_ array shrinks as elements
are removed from the map.