src/vs/platform/contextkey/common/contextkey.ts/ContextKeyInExpr/evaluate
class ContextKeyInExpr:	public evaluate(context: IContext): boolean {
		const source = context.getValue(this.valueKey);

		const item = context.getValue(this.key);

		if (Array.isArray(source)) {
			return source.includes(item as any);
		}

		if (typeof item === 'string' && typeof source === 'object' && source !== null) {
			return hasOwnProperty.call(source, item);
		}
		return false;
	}