sortedBy(keySelector) {
        return seq(this.toArray().sort((e1, e2) => {
            let out1 = keySelector(e1);
            let out2 = keySelector(e2);
            return out1 < out2 ? -1 :
                   out1 > out2 ? +1 :
                   0;
        }));
    }
Returns a sequence containing the same items, but in ascending order of outputs from the given function.
@returns {!Seq.<T>}
@template T