方法 'MultiTrackTween' 没有为类型 'FadeAnimation' 定义

Question

import 'package:flutter/material.dart';
import 'package:flutter/animation.dart';

class FadeAnimation extends StatelessWidget {
  final double delay;
  final Widget child;

  FadeAnimation(this.delay, this.child);

  ///build animation track
  @override
  Widget build(BuildContext context) {
    final tween = MultiTrackTween([
      Track("opacity")
          .add(Duration(milliseconds: 500), Tween(begin: 0.0, end: 1.0)),
      Track("translateY").add(
          Duration(milliseconds: 500), Tween(begin: -30.0, end: 0.0),
          curve: Curves.easeOut)
    ]);

    /// fade animation controller
    return ControlledAnimation(
      delay: Duration(milliseconds: (500 * delay).round()),
      duration: tween.duration,
      tween: tween,
      child: child,
      builderWithChild: (context, child, animation) => Opacity(
        opacity: animation["opacity"],
        child: Transform.translate(
            offset: Offset(0, animation["translateY"]), child: child),
      ),
    );
  }
}

当我编写此代码时，它显示未为类型“FadeAnimation”定义“MultiTrackTween”方法。 Track、ControlledAnimation 也显示相同的错误。请帮忙。