src/vs/platform/markers/common/markerService.ts/MarkerService/_merge
class MarkerService:	private static _merge(all: (readonly URI[])[]): URI[] {
		const set = new ResourceMap<boolean>();
		for (let array of all) {
			for (let item of array) {
				set.set(item, true);
			}
		}
		return Array.from(set.keys());
	}