src/vs/workbench/contrib/extensions/browser/fileBasedRecommendations.ts/FileBasedRecommendations/filterInstalled
class FileBasedRecommendations:	private filterInstalled(recommendationsToSuggest: string[], installed: IExtension[]): string[] {
		const installedExtensionsIds = installed.reduce((result, i) => {
			if (i.enablementState !== EnablementState.DisabledByExtensionKind) {
				result.add(i.identifier.id.toLowerCase());
			}
			return result;
		}, new Set<string>());
		return recommendationsToSuggest.filter(id => !installedExtensionsIds.has(id.toLowerCase()));
	}