anchor.diff: -import 'dart:html';
  +// Copyright 2019 The Chromium Authors. 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:html' hide VoidCallback;
   import 'dart:js';
   
   import 'package:meta/meta.dart';
  
positive.code: 
    _ChunkPosition get position => _position;
    _ChunkPosition _position;
    set position(_ChunkPosition p) {
      if (_position == p) return;
      _position = p;
  
negative.code:   Rect _renderedViewport;
    Rect _viewport = Rect.zero;
  
    void _dispatchMouseMoveEvent() {
      if (_onMouseMove != null) {
        _onMouseMove(_clientToGlobal(_currentMouseHover));
  