Does the same thing as https://github.com/hackcatml/frida-flutterproxy

flutter_proxy.js

var PROXY_IP = "127.0.0.1";
var PROXY_PORT = 8080;

var proxyBytes = PROXY_IP.split(".").map(Number);
var proxyPortHi = (PROXY_PORT >> 8) & 0xff;
var proxyPortLo = PROXY_PORT & 0xff;

var connect = Module.getExportByName("libc.so", "connect");

Interceptor.attach(connect, {
    onEnter: function (args) {
        this.fd = args[0].toInt32();
        this.addr = args[1];
        var family = this.addr.readU16();

        if (family !== 2) return; // AF_INET only

        var port = (this.addr.add(2).readU8() << 8) | this.addr.add(3).readU8();
        var ip = this.addr.add(4).readU8() + "." +
                 this.addr.add(5).readU8() + "." +
                 this.addr.add(6).readU8() + "." +
                 this.addr.add(7).readU8();

        if (ip === PROXY_IP && port === PROXY_PORT) return;
        if (ip === "127.0.0.1") return;
        // if (port !== 80 && port !== 443 && port !== 8083 && port !== 8443) return;

        send("[proxy] " + ip + ":" + port + " -> " + PROXY_IP + ":" + PROXY_PORT);

        this.addr.add(2).writeU8(proxyPortHi);
        this.addr.add(3).writeU8(proxyPortLo);
        this.addr.add(4).writeU8(proxyBytes[0]);
        this.addr.add(5).writeU8(proxyBytes[1]);
        this.addr.add(6).writeU8(proxyBytes[2]);
        this.addr.add(7).writeU8(proxyBytes[3]);
    }
});

send("[*] connect() hook active -> " + PROXY_IP + ":" + PROXY_PORT);
send("[*] Configure Burp/mitmproxy in invisible/transparent proxy mode");