Usecase

Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(title: Text('IntrinsicWidth')),
    body: Center(
      child: IntrinsicHeight(
        child: Row(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            RaisedButton(
              onPressed: () {},
              child: Text('Short'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('A bit Longer'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('The Longest text button'),
            ),
          ],
        ),
      ),
    ),
  );
}

products.sort((a,b) => a.compareTo(b));

sdk: ">=2.12.0 <3.0.0"


Then follow the steps:

Run flutter upgrade in the terminal to upgrade Flutter
Run dart migrate to run the dart migration tool.
Solve all errors which the migration tool shows.
Run flutter pub outdated --mode=null-safety to print all outdated packages.
Run flutter pub upgrade --null-safety to upgrade all packages automatically.
Check the code for errors and solve them (Very important).
Run dart migrate again and it should now be successful. Follow the link to checkout the proposed changes.
Press the "Apply Migration" button.
Check the code for errors again and fix them.

import 'package:flutter/material.dart';

void main() {
  runApp(
    new MaterialApp(
      title: 'Hello World App',
      home: new myApp(),
    )
  );
}

class myApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Hello World App'),
      ),
      body: new Center(
        child: new Text(
          'Hello, world!'
        ),
      ),
    );
  }
}

MaterialApp(
 debugShowCheckedModeBanner: false
)

Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(title: Text('IntrinsicWidth')),
    body: Center(
      child: IntrinsicWidth(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            RaisedButton(
              onPressed: () {},
              child: Text('Short'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('A bit Longer'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('The Longest text button'),
            ),
          ],
        ),
      ),
    ),
  );
}

<uses-permission android:name="android.permission.MANAGE_EXTERNAL_STORAGE"
        tools:ignore="ScopedStorage" />

String capitalize(String s) {
  if (s == null || s.isEmpty) {
    return s;
  }
  return s.length < 1 ? s.toUpperCase() : s[0].toUpperCase() + s.substring(1);
}

Text("Border test",
    style: TextStyle(
      inherit: true,
      fontSize: 48.0,
      color: Colors.pink,
      shadows: [
        Shadow( // bottomLeft
          offset: Offset(-1.5, -1.5),
          color: Colors.white
        ),
        Shadow( // bottomRight
          offset: Offset(1.5, -1.5),
          color: Colors.white
        ),
        Shadow( // topRight
          offset: Offset(1.5, 1.5),
          color: Colors.white
        ),
        Shadow( // topLeft
          offset: Offset(-1.5, 1.5),
          color: Colors.white
        ),
      ]
    ),
);

Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(title: Text('IntrinsicWidth')),
    body: Center(
      child: IntrinsicWidth(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            RaisedButton(
              onPressed: () {},
              child: Text('Short'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('A bit Longer'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('The Longest text button'),
            ),
          ],
        ),
      ),
    ),
  );
}

  class MyBlinkingButton extends StatefulWidget {
    @override
    _MyBlinkingButtonState createState() => _MyBlinkingButtonState();
  }

  class _MyBlinkingButtonState extends State<MyBlinkingButton>
      with SingleTickerProviderStateMixin {
    AnimationController _animationController;

    @override
    void initState() {
      _animationController =
          new AnimationController(vsync: this, duration: Duration(seconds: 1));
      _animationController.repeat(reverse: true);
      super.initState();
    }

    @override
    Widget build(BuildContext context) {
      return FadeTransition(
        opacity: _animationController,
        child: MaterialButton(
          onPressed: () => null,
          child: Text("Text button"),
          color: Colors.green,
        ),
      );
    }

    @override
    void dispose() {
      _animationController.dispose();
      super.dispose();
    }
  }

                Container(
                  height: SizeConfig.safeBlockVertical * 24,
                  width: SizeConfig.safeBlockHorizontal * 45,
                  padding: EdgeInsets.only(
                      left: SizeConfig.safeBlockHorizontal * 5,
                      right: SizeConfig.safeBlockHorizontal * 4,
                      bottom: SizeConfig.safeBlockVertical * 2),
                  decoration: BoxDecoration(
                      borderRadius: BorderRadius.circular(20),
                      color: Colors.white,
                      gradient: RadialGradient(
                          focalRadius: 100,
                          radius: 1.5,
                          center: Alignment.bottomLeft,
                          colors: [
                            colorStyles["maslowGreen"]!.withOpacity(0.65),
                            colorStyles["maslowGreen"]!.withOpacity(0.65),
                          ],
                          stops: [
                            0.0,
                            1.0
                          ])),
                  child: _collectionName(),
                ),

Container(
                                child: Text(
                                  items[index].itemName,
                                  style: TextStyle(
                                      color: Colors.black, fontSize: 30),
                                  textAlign: TextAlign.center,
                                ), //Text
                                height: 40,
                                width: 400,
                                  decoration:
                               BoxDecoration(
                  border: Border.all(color: Colors.black38, width: 3),
                  borderRadius: BorderRadius.circular(20),
                  boxShadow: [
                    BoxShadow(
                      color: Colors.black.withOpacity(0.5),
                      spreadRadius: 5,
                      blurRadius: 5,
                      offset: Offset(0, 3), // changes position of shadow
                    ),
                  ],
                  color: Colors.white,
                  image: DecorationImage(
                    image: AssetImage(filterList[index].itemImg),
                    fit: BoxFit.contain,
                  ),
                ),

hasSpecialCharacters = password.contains(new RegExp(r'[!@#$%^&*(),.?":{}|<>]')); 

  final bool isNumeric = password.contains(RegExp('[0-9]'));
  final bool isLowerCase = password.contains(RegExp("(?:[^a-z]*[a-z]){1}"));
  final bool isUpperCase = password.contains(RegExp("(?:[^A-Z]*[A-Z]){1}"));