Clear analyzer warning on Observatory

runtime/observatory/lib/src/elements/sample_buffer_control.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:html';
 import 'package:observatory/models.dart' as M;
 import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
 import 'package:observatory/src/elements/helpers/tag.dart';
-import 'package:observatory/src/elements/helpers/uris.dart';
 import 'package:observatory/utils.dart';
 
 class SampleBufferControlChangedElement {
   final SampleBufferControlElement element;
   SampleBufferControlChangedElement(this.element);
 }
 
 class SampleBufferControlElement extends HtmlElement implements Renderable {
   static const tag =
       const Tag<SampleBufferControlElement>('sample-buffer-control');
@@ skipping 26 matching lines @@
   set showTag(bool value) => _showTag = _r.checkAndReact(_showTag, value);
   set profileVM(bool value) => _profileVM = _r.checkAndReact(_profileVM, value);
 
   factory SampleBufferControlElement(
       M.VM vm,
       M.SampleProfileLoadingProgress progress,
       Stream<M.SampleProfileLoadingProgressEvent> progressStream,
       {M.SampleProfileTag selectedTag: M.SampleProfileTag.none,
       bool showTag: true,
       RenderingQueue queue}) {
-    assert(vm != null);
     assert(progress != null);
     assert(progressStream != null);
     assert(selectedTag != null);
     assert(showTag != null);
     SampleBufferControlElement e = document.createElement(tag.name);
     e._r = new RenderingScheduler(e, queue: queue);
     e._vm = vm;
     e._progress = progress;
     e._progressStream = progressStream;
     e._tag = selectedTag;
@@ skipping 194 matching lines @@
   }
 
   void _enableProfiler() {
     _vm.enableProfiler().then((_) {
       _triggerModeChange(_toEvent(null));
     });
   }
 
   void _triggerModeChange(e) => _onTagChange.add(e);
 }