src/vs/workbench/services/decorations/browser/decorationsService.ts/DecorationsService/_keepItem
class DecorationsService:	private _keepItem(map: DecorationEntry, provider: IDecorationsProvider, uri: URI, data: IDecorationData | undefined): IDecorationData | null {
		const deco = data ? data : null;
		const old = map.set(provider, deco);
		if (deco || old) {
			// only fire event when something changed
			this._onDidChangeDecorationsDelayed.fire(uri);
		}
		return deco;
	}