src/vs/workbench/services/decorations/browser/decorationsService.ts/DecorationRule/keyOf
class DecorationRule:	static keyOf(data: IDecorationData | IDecorationData[]): string {
		if (Array.isArray(data)) {
			return data.map(DecorationRule.keyOf).join(',');
		} else {
			const { color, letter } = data;
			if (ThemeIcon.isThemeIcon(letter)) {
				return `${color}+${letter.id}`;
			} else {
				return `${color}/${letter}`;
			}
		}
	}