solidify() {
        let knownSolidTypes = [
            Float32Array,
            Float64Array,
            Int8Array,
            Int16Array,
            Int32Array,
            Uint8Array,
            Uint16Array,
            Uint32Array
        ];

        if (Array.isArray(this._iterable)) {
            return this;
        }
        for (let t of knownSolidTypes) {
            if (this._iterable instanceof t) {
                return this;
            }
        }
        return new Seq(this.toArray());
    }
Returns a sequence with the same items, but precomputed and stored. If the sequence is already solid, e.g. it is
backed by an array, then it is returned directly (and unchanged).
@returns {!Seq.<T>}
@template T