static repeat(item, repeatCount) {
        if (repeatCount < 0) {
            throw new Error("needed repeatCount >= 0");
        }

        return Seq.fromGenerator(function*() {
            for (let i = 0; i < repeatCount; i++) {
                yield item;
            }
        })
    }
Returns a sequence of the same item repeated the given number of times.
@param {T} item
@param {!int} repeatCount
@returns {!Seq.<T>}
@template T