tryPeekCount() {
        if (Array.isArray(this._iterable) || !GENERIC_ARRAY_TYPES.every(t => !(this._iterable instanceof t))) {
            return this._iterable.length;
        }
        if (this._iterable instanceof Map || this._iterable instanceof Set) {
            return this._iterable.size;
        }
        return undefined;
    }
If the sequence is of a known type with a known number of items, then returns the length of the sequence.
Otherwise, returns undefined.
It is guaranteed that the sequence will not be iterated by this method.
@returns {!int|undefined}