Charles Proxy with Flutter apps examples. See https://medium.com/hoursofoperation/use-charles-proxy-to-debug-network-requests-in-flutter-apps-2f2083275cad

custom_proxy.dart

import 'dart:io';
import 'package:flutter/foundation.dart';

/// Allows you to set and enable a proxy for your app
class CustomProxy {
  /// A string representing an IP address for the proxy server
  final String ipAddress;

  /// The port number for the proxy server
  /// Can be null if port is default.
  final int port;

  /// Set this to true
  /// - Warning: Setting this to true in production apps can be dangerous. Use with care!
  bool allowBadCertificates;

  /// Initializer
  CustomProxy(
      {@required this.ipAddress, this.port, this.allowBadCertificates = false});

  /// Initializer from string
  /// Note: Uses static method, rather than named init to allow final properties.
  static CustomProxy fromString({@required String proxy}) {
    // Check if valid
    if (proxy == null || proxy == "") {
      assert(
          false, "Proxy string passed to CustomProxy.fromString() is invalid.");
      return null;
    }

    // Build and return
    final proxyParts = proxy.split(":");
    final _ipAddress = proxyParts[0];
    final _port = proxyParts.length > 0 ? int.tryParse(proxyParts[1]) : null;
    return CustomProxy(
      ipAddress: _ipAddress,
      port: _port,
    );
  }

  /// Enable the proxy
  void enable() {
    HttpOverrides.global =
        new CustomProxyHttpOverride.withProxy(this.toString());
  }

  /// Disable the proxy
  void disable() {
    HttpOverrides.global = null;
  }

  @override
  String toString() {
    String _proxy = this.ipAddress;
    if (this.port != null) {
      _proxy += ":" + this.port.toString();
    }
    return _proxy;
  }
}

custom_proxy_http_override.dart

import 'dart:io';
import 'package:flutter/foundation.dart';

/// This class overrides the global proxy settings.
class CustomProxyHttpOverride extends HttpOverrides {
  /// The entire proxy server
  /// Format: "localhost:8888"
  final String proxyString;

  /// Set this to true
  /// - Warning: Setting this to true in production apps can be dangerous. Use with care!
  final bool allowBadCertificates;

  /// Initializer
  CustomProxyHttpOverride.withProxy(
    this.proxyString, {
    this.allowBadCertificates = false,
  });

  /// Override HTTP client creation
  @override
  HttpClient createHttpClient(SecurityContext context) {
    return super.createHttpClient(context)
      ..findProxy = (uri) {
        assert(this.proxyString != null && this.proxyString.isNotEmpty,
            'You must set a valid proxy if you enable it!');
        return "PROXY " + this.proxyString + ";";
      }
      ..badCertificateCallback =
          this.allowBadCertificates ? (X509Certificate cert, String host, int port) => true : null;
  }
}