src/vs/workbench/browser/parts/editor/editorsObserver.ts/EditorsObserver/ensureKey
class EditorsObserver:	private ensureKey(group: IEditorGroup, editor: IEditorInput): IEditorIdentifier {
		let groupMap = this.keyMap.get(group.id);
		if (!groupMap) {
			groupMap = new Map();

			this.keyMap.set(group.id, groupMap);
		}

		let key = groupMap.get(editor);
		if (!key) {
			key = { groupId: group.id, editor };
			groupMap.set(editor, key);
		}

		return key;
	}