src/vs/platform/markers/common/markerService.ts/DoubleResourceMap/delete
class DoubleResourceMap:	delete(resource: URI, owner: string): boolean {
		let removedA = false;
		let removedB = false;
		const ownerMap = this._byResource.get(resource);
		if (ownerMap) {
			removedA = ownerMap.delete(owner);
		}
		const resourceMap = this._byOwner.get(owner);
		if (resourceMap) {
			removedB = resourceMap.delete(resource);
		}
		if (removedA !== removedB) {
			throw new Error('illegal state');
		}
		return removedA && removedB;
	}