A recreation of curly-brackets.html on my formatting-help repository.

curly-brackets.dart

import "dart:html";

final input = querySelector("#input") as TextAreaElement;
final output = querySelector("#output") as TextAreaElement;
final btn = querySelector("#button") as ButtonElement;
final radio = querySelector("#sameline") as RadioButtonInputElement;
// Setup elements

void main() {
    btn.addEventListener("click", (Event event) {
        if (radio.checked as bool) { 
            output.value = input.value?.replaceAll(RegExp(" *\n *{"), " {");
            // Remove the newline
        } else {
            output.value = input.value?.replaceAllMapped(
                RegExp(r"^.*{\s*$", multiLine: true),
                (n) {
                    var m = (n[0] as String).trimRight(); // m stands for match
                    return (
                        m.substring(0, m.length - 1) + "\n" +
                        (RegExp(r"^\s?").firstMatch(m)![0]! * 
                        RegExp(r"^\s*").firstMatch(m)![0]!.length) +
                        "{"
                    );
                    // All we do is return a curly bracket on a new line
                    // That's indented on the same level as the last statement
                }
            );
        }
    }); // Onclick event for the button
    
    input.focus(); // Instantly focus on the input area
}

index.html

<h1>Change placement of curly brackets</h1>
<h2>Are you ever annoyed when someone places their curly brackets/braces on a newline, <br>but you like them on the same line?</h2>
<p>Well this is for you!</p>
<textarea id="input" placeholder="Input text here..."></textarea>
<button id="button">Change curly bracket placement</button>
<br>
<div>
    <input type="radio" name="select" id="sameline" checked>
    <label for="sameline">Move curly bracket to same line</label>
</div>
<div>
    <input type="radio" name="select" id="newline">
    <label for="newline">Move curly bracket to new line</label>
</div>
<br>
<textarea id="output" placeholder="Output here."></textarea>
<style>
    textarea {
        width: 500px;
        height: 200px;
    }
    body {
        margin: 10px;
        font-family: sans-serif;
        tab-size: 4;
        -moz-tab-size: 4;
        -webkit-tab-size: 4;
    }
</style>

https://dartpad.dev/6081062503e8b6154177ff1eb7ab1d21?null_safety=true

Made with ❤️ by Dartpad, the official online IDE/code runner for Dart, which allows even HTML to be used.