src/vs/workbench/browser/parts/editor/editorsObserver.ts/EditorsObserver/findKey
class EditorsObserver:	private findKey(group: IEditorGroup, editor: IEditorInput): IEditorIdentifier | undefined {
		const groupMap = this.keyMap.get(group.id);
		if (!groupMap) {
			return undefined;
		}

		return groupMap.get(editor);
	}