tag() {
        let tag = this.uint32(), fieldNo = tag >>> 3, wireType = tag & 7;
        if (fieldNo <= 0 || wireType < 0 || wireType > 5)
            throw new Error("illegal tag: field no " + fieldNo + " wire type " + wireType);
        return [fieldNo, wireType];
    }
Reads a tag - field number and wire type.