isBitSet(i) {
    e2e.assert(
        (i >= 0 && i < TREE_SIZE_), 'index ' + i + ' out of range',
        InvalidArgumentsError);
    return Boolean(this.indexBytes_[i >> 3] & MASKS_[i % 8]);
  }
Determines whether the i'th bit of the index is set (with the MSB being
i = 0).

@param {number} i
@return {boolean}
@throws {!InvalidArgumentsError} If the value of i is incorrect.