src/vs/base/common/keybindingLabels.ts/ModifierLabelProvider/toLabel
class ModifierLabelProvider:	public toLabel<T extends Modifiers>(OS: OperatingSystem, parts: T[], keyLabelProvider: KeyLabelProvider<T>): string | null {
		if (parts.length === 0) {
			return null;
		}

		const result: string[] = [];
		for (let i = 0, len = parts.length; i < len; i++) {
			const part = parts[i];
			const keyLabel = keyLabelProvider(part);
			if (keyLabel === null) {
				// this keybinding cannot be expressed...
				return null;
			}
			result[i] = _simpleAsString(part, keyLabel, this.modifierLabels[OS]);
		}
		return result.join(' ');
	}