src/vs/workbench/browser/parts/editor/editorPart.ts/EditorPart/doRemoveGroupWithEditors
class EditorPart:	private doRemoveGroupWithEditors(groupView: IEditorGroupView): void {
		const mostRecentlyActiveGroups = this.getGroups(GroupsOrder.MOST_RECENTLY_ACTIVE);

		let lastActiveGroup: IEditorGroupView;
		if (this._activeGroup === groupView) {
			lastActiveGroup = mostRecentlyActiveGroups[1];
		} else {
			lastActiveGroup = mostRecentlyActiveGroups[0];
		}

		// Removing a group with editors should merge these editors into the
		// last active group and then remove this group.
		this.mergeGroup(groupView, lastActiveGroup);
	}