findGateCoveringSlot(col, row) {
        let key = col+":"+row;
        if (!this._gateSlotCoverMap.has(key)) {
            return undefined;
        }
        return this._gateSlotCoverMap.get(key);
    }
A slot is covered when a gate is in it or extends over it.
@param {!int} col
@param {!int} row
@returns {undefined|!{col: !int, row: !int, gate: !Gate}}