src/vs/workbench/common/editor/editorGroupModel.ts/EditorGroupModel/closeEditor
class EditorGroupModel:	closeEditor(candidate: EditorInput, openNext = true): EditorInput | undefined {
		const event = this.doCloseEditor(candidate, openNext, false);

		if (event) {
			this._onDidCloseEditor.fire(event);

			return event.editor;
		}

		return undefined;
	}