A recreation of replace-indentation.html on my formatting-help repository.

_replace-indentation.dart

import "dart:html";

final input = querySelector("#input") as TextAreaElement;
final output = querySelector("#output") as TextAreaElement;
final button = querySelector("#format") as ButtonElement;
final number = querySelector("#spaces") as InputElement;
final oldNumber = querySelector("#oldspaces") as InputElement;
// Setup the variables

void main() {
    input.focus(); // Allow instant pasting of code
    
    final inputOnInput = (Event event) {
        final String? match =
            RegExp(r"(^|\n) +").firstMatch(input.value as String)?[0];

        oldNumber.value = (match == null
            ? 2
            : (match.length - (
                RegExp("^ +").hasMatch(input.value!.split("\n")[0]) ? 0 : 1))
        ).toString();
        number.value = oldNumber.value == "4" ? "2" : "4";
    }; // We make this a variable so we can call it
    
    input.addEventListener("input", inputOnInput); // Add the event listener for the input element
    
    inputOnInput(Event("input")); // We want to call the function automatically
    
    button.addEventListener("click", (event) {
        output.value = (input.value?.replaceAllMapped(
            RegExp(r"(\n|^) +"), // Matches newline/string-start, then a positive number of spaces
            (n) =>
                "\n" + // we still want a newline
                " " * (n[0]!.substring(1).length *
                    int.parse(number.value!) /
                    int.parse(oldNumber.value!)
                    // Mathy stuff that gets the right amount of indentation
                ).floor() // We can't multiply a string by a decimal number
            )
        );
    });
}

index.html

<style>
    textarea {
        width: 500px;
        height: 200px;
    }

    html {
        color: white;
        font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", "Google Sans", Roboto, Ubuntu, sans-serif;
    }
</style>
<h1>Do you have to deal with formatters that give you an indentation level you don't like?</h1>
<h2>Well this is the tool for you!</h2>
<p>Note: doesnt work for tabs</p>
<p>Just paste your code into the top area, set the amount of spaces you want to get to, and click the button!</p>
<textarea id="input"></textarea>
<br>
<button id="format">Format with spaces</button>
<br>
From <input id="oldspaces" type="number"> spaces to <input id="spaces" type="number"> spaces 
<br>
<textarea id="output"></textarea>

https://dartpad.dev/5d84d9ad08adbf16afc57b847e443e23?null_safety=true

Created with ❤️ by Dartpad, which allows you to run small Dart applications, even interacting with HTML.