builder.put("foo", 2);
    assertMapEquals(builder.buildOrThrow(), "foo", 2);
  }

  // for GWT compatibility
  static class SimpleEntry<K, V> extends AbstractMapEntry<K, V> {
    public K key;
    public V value;

    SimpleEntry(K key, V value) {
      this.key = key;
      this.value = value;
    }

    @Override
    public K getKey() {
      return key;
    }