getUnmetContextKeys() {
        let result = new Set(this._requiredContextKeys);
        if (this.knownCircuit !== undefined) {
            for (let key of this.knownCircuit.getUnmetContextKeys()) {
                result.add(key);
            }
        }
        return result;
    }
Returns context keys required by this gate, including keys inherited from its custom circuit (if applicable).
@returns {!Set.<!String>}