src/vs/workbench/browser/parts/editor/editorsObserver.ts/EditorsObserver/removeMostRecentEditor
class EditorsObserver:	private removeMostRecentEditor(group: IEditorGroup, editor: IEditorInput): void {

		// Update in resource map
		this.updateEditorResourcesMap(editor, false);

		// Update in MRU list
		const key = this.findKey(group, editor);
		if (key) {

			// Remove from most recent editors
			this.mostRecentEditorsMap.delete(key);

			// Remove from key map
			const map = this.keyMap.get(group.id);
			if (map && map.delete(key.editor) && map.size === 0) {
				this.keyMap.delete(group.id);
			}

			// Event
			this._onDidMostRecentlyActiveEditorsChange.fire();
		}
	}