Fix import failure caused by #6266. (#6272)

Author: bmd3k

tensorboard/webapp/feature_flag/views/feature_flag_modal_trigger_container.ts

@@ -16,7 +16,8 @@ import {ComponentType} from '@angular/cdk/overlay';
 import {Component, OnDestroy, OnInit} from '@angular/core';
 import {MatDialog, MatDialogRef} from '@angular/material/dialog';
 import {Store} from '@ngrx/store';
-import {Subject, takeUntil} from 'rxjs';
+import {Subject} from 'rxjs';
+import {takeUntil} from 'rxjs/operators';
 import {State} from '../../app_state';
 import {featureFlagOverridesReset} from '../actions/feature_flag_actions';
 import {getShowFlagsEnabled} from '../store/feature_flag_selectors';