src/vs/workbench/contrib/debug/browser/loadedScriptsView.ts/BaseTreeItem/createIfNeeded
class BaseTreeItem:	createIfNeeded<T extends BaseTreeItem>(key: string, factory: (parent: BaseTreeItem, label: string) => T): T {
		let child = <T>this._children.get(key);
		if (!child) {
			child = factory(this, key);
			this._children.set(key, child);
		}
		return child;
	}