src/vs/base/common/map.ts/ResourceMap/constructor
class ResourceMap:	constructor(mapOrKeyFn?: ResourceMap<T> | ResourceMapKeyFn, toKey?: ResourceMapKeyFn) {
		if (mapOrKeyFn instanceof ResourceMap) {
			this.map = new Map(mapOrKeyFn.map);
			this.toKey = toKey ?? ResourceMap.defaultToKey;
		} else {
			this.map = new Map();
			this.toKey = mapOrKeyFn ?? ResourceMap.defaultToKey;
		}
	}