Google SignIn Button:

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'),
            ),
          ],
        ),
      ),
    ),
  );
}

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'),
            ),
          ],
        ),
      ),
    ),
  );
}

MaterialApp(
 debugShowCheckedModeBanner: false
)

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!'
        ),
      ),
    );
  }
}

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

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Run flutter pub outdated --mode=null-safety to print all outdated packages.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.

Future<File> moveFile(File sourceFile, String newPath) async {
  try {
    // prefer using rename as it is probably faster
    return await sourceFile.rename(newPath);
  } on FileSystemException catch (e) {
    // if rename fails, copy the source file and then delete it
    final newFile = await sourceFile.copy(newPath);
    await sourceFile.delete();
    return newFile;
  }
}

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

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

android {
    defaultConfig {
        ...
        multiDexEnabled true
    }
    ...
}

dependencies {
  implementation 'com.android.support:multidex:1.0.3'
}

String shakespeareQuote = "All the world's a stage, ...";

import 'package:path_provider/path_provider.dart';
Future<void> _deleteCacheDir() async {
final cacheDir = await getTemporaryDirectory();
if (cacheDir.existsSync()) {
cacheDir.deleteSync(recursive: true);}}
final appDir = await getApplicationSupportDirectory();
if (appDir.existsSync()) {
appDir.deleteSync(recursive: true);}}

                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(),
                ),

InputDecoration(
   isDense: true,
   prefixIcon:Text("\$"),
   prefixIconConstraints: BoxConstraints(minWidth: 0, minHeight: 0),
),