src/vs/platform/undoRedo/common/undoRedoService.ts/WorkspaceStackElement/setValid
class WorkspaceStackElement:	public setValid(resourceLabel: string, strResource: string, isValid: boolean): void {
		if (isValid) {
			if (this.invalidatedResources) {
				this.invalidatedResources.delete(strResource);
				if (this.invalidatedResources.size === 0) {
					this.invalidatedResources = null;
				}
			}
		} else {
			if (!this.invalidatedResources) {
				this.invalidatedResources = new RemovedResources();
			}
			if (!this.invalidatedResources.has(strResource)) {
				this.invalidatedResources.set(strResource, new ResourceReasonPair(resourceLabel, RemovedResourceReason.ExternalRemoval));
			}
		}
	}