Whats hapning

import javax.swing.*;  
public class FirstSwingExample {  
public static void main(String[] args) {  
JFrame f=new JFrame();//creating instance of JFrame  
          
JButton b=new JButton("click");//creating instance of JButton  
b.setBounds(130,100,100, 40);//x axis, y axis, width, height  
          
f.add(b);//adding button in JFrame  
          
f.setSize(400,500);//400 width and 500 height  
f.setLayout(null);//using no layout managers  
f.setVisible(true);//making the frame visible  
}  
}  

/* Box sizing rules */
*,
*::before,
*::after {
  box-sizing: border-box;
}

/* Remove default margin */
body,
h1,
h2,
h3,
h4,
p,
figure,
blockquote,
dl,
dd {
  margin: 0;
}

/* Remove list styles on ul, ol elements with a list role, which suggests default styling will be removed */
ul[role='list'],
ol[role='list'] {
  list-style: none;
}

/* Set core root defaults */
html:focus-within {
  scroll-behavior: smooth;
}

/* Set core body defaults */
body {
  min-height: 100vh;
  text-rendering: optimizeSpeed;
  line-height: 1.5;
}

/* A elements that don't have a class get default styles */
a:not([class]) {
  text-decoration-skip-ink: auto;
}

/* Make images easier to work with */
img,
picture {
  max-width: 100%;
  display: block;
}

/* Inherit fonts for inputs and buttons */
input,
button,
textarea,
select {
  font: inherit;
}

/* Remove all animations, transitions and smooth scroll for people that prefer not to see them */
@media (prefers-reduced-motion: reduce) {
  html:focus-within {
   scroll-behavior: auto;
  }
  
  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}

// in css:
/* Display line under clicked navbar link */
.active {
  text-decoration-line: underline !important;
  text-decoration-thickness: 2px !important;
  color: rgb(20, 19, 19);
}

//in html: 
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>

    <script>
      $(document).ready(function () {
        // Underline to remain in navbar after click using URL
        jQuery(function ($) {
          var path = window.location.href; // because the 'href' property of the DOM element is the absolute path
          $('.nav-link').each(function () {
            if (this.href === path) {
              $(this).addClass('active');
            }
          });
        });
      });
    </script>

//Note class in link should be nav-link

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

/* slightly transparent fallback */
.backdrop-blur {
  background-color: rgba(255, 255, 255, .9);
}

/* if backdrop support: very transparent and blurred */
@supports ((-webkit-backdrop-filter: blur(2em)) or (backdrop-filter: blur(2em))) {
  .backdrop-blur {
    background-color: rgba(255, 255, 255, .5);
    -webkit-backdrop-filter: blur(2em);
    backdrop-filter: blur(2em);
  }
}

body{
    background-image: url("img_tree.gif");
    background-repeat: no-repeat;
    background-attachment: fixed;
}

#banner .slick-dots {
	display: flex;
	position: absolute;
	left: 50%;
	bottom: 30px;
	transform: translateX(-50%);
}

#banner .slick-dots li button {
	font-size: 0;
	width: 16px;
	height: 16px;
	border: 1px solid #fff;
	background: transparent;
	border-radius: 50%;
	margin: 0 5px;
}

#banner .slick-dots li.slick-active button {
	width: 20px;
	height: 20px;
	background: var(--hover);
	border-color: var(--hover);
}

let hijo = document.querySelector('.hijo');
let etiqueta = document.querySelectorAll('.etiqueta');
//El metodo .getBoundingClientRect() nos da 
//la posicion de un elemento con respecto al viewport(en numeros).
// tomare la posicion constante del elemento .hijo 
const coords = hijo.getBoundingClientRect();
const j = coords.top; // estamos obteniendo la posicion top en numero
const k = coords.left;

// se aplica forEach para obtener posicion de cada movimiento del mouse
etiqueta.forEach(link => {
// se obtiene la posicion del mouse con la funcion  mover(e)
    function mover(e){
// se calcula la posicion top y left del mouse al cual le restamos
// la posicion del elemento "hijo"; resultado: mouse e "hijo" juntos 
	 let m = e.pageY ;
	 let n = e.pageX ;

	 let t = m - j;
	 let l = n - k;
	 hijo.style.top = t + "px"; // agregando px a los numeros
	 hijo.style.left = l + "px";
    };
// mouseover dice que entro en elemento etiqueta
    link.addEventListener("mouseover",() => {
	hijo.classList.add("edd1");// solo agrega un class que le da color red

	 window.addEventListener("mousemove",mover);// agrega el evento junto a la funcion 
    });

    link.addEventListener("mouseleave",() => {// mouseleave dice que salio del elemento etiqueta
	hijo.classList.remove("edd1"); // le quita un class, que le quita color red
	 window.removeEventListener("mousemove", mover);// remuev el evento
    });
});

/* Simples estilos para delimitar los div*/
.box{
width: 300px;
height: 300px;
border: solid 1px black;
}
.etiqueta{
width:100px;
height:100px;
position: absolute;
left:50px;
top:50px;
background: green;
overflow: hidden;
position:relative;
}
.hijo{
position: absolute;
width: 20px;
height: 20px;
border-radius: 50%;
background-color: blue;
transform: scale(0); /*desaparece el elemento hijo*/
}
.edd1{
background-color: red;
transform: scale(1); /*reaparece el elemento hijo con el mause al centro*/
} 

// un ejemplo con html
<div class="box">
  <div class="etiqueta">
  <div class="hijo"></div>
  </div>
</div> 

const header = document.querySelector('header');
let lastScroll = 0;
const scrollThreshold = 10; 

window.addEventListener('scroll', () => {
    const currentScroll = window.scrollY;

    if (currentScroll > lastScroll && currentScroll > scrollThreshold) {
        header.classList.add('small');
    } else if (currentScroll === 0) {
        header.classList.remove('small');
    }

    lastScroll = currentScroll;
});

const hamburger = document.querySelector(".hamburger");
const navmenu = document.querySelector(".nav-menu");

hamburger.addEventListener("click", () => {
    hamburger.classList.toggle("active");
    navmenu.classList.toggle("active");
});

document.querySelectorAll(".nav-link").forEach(n => n.addEventListener("click", () => {
    hamburger.classList.remove("active");
    navmenu.classList.remove("active");
}));

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.

img {
  mask-image: url(‘mask.png’) linear-gradient(-45deg,
                        rgba(0,0,0,1) 20%, rgba(0,0,0,0) 50%);
  mask-image: url(#masking); /*referencing to the element generated and defined in SVG code*/
} 

<style>

 .circle:before {
                    content: ' \25CF';
                    font-size: 50px;
                    color: black;
                }
</style>

<span class="circle"></span>

.wrapper {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  grid-gap: 10px;
  grid-auto-rows: minmax(100px, auto);
}
.one {
  grid-column: 1 / 3;
  grid-row: 1;
}
.two { 
  grid-column: 2 / 4;
  grid-row: 1 / 3;
}
.three {
  grid-column: 1;
  grid-row: 2 / 5;
}
.four {
  grid-column: 3;
  grid-row: 3;
}
.five {
  grid-column: 2;
  grid-row: 4;
}
.six {
  grid-column: 3;
  grid-row: 4;
}

.c-hero__title {
  max-width: 36rem;
  text-wrap: balance;
}