Firebase Remote Config - web implementation

conditional_import.dart

/// Base interface
import 'remote_config_base.dart';

/// Conditional import
import 'remote_config_mobile.dart'
  // ignore: uri_does_not_exist
  if (dart.library.html) 'remote_config_web.dart';

...
  final config = await getRemoteConfig();
...

index.html

...
  <!-- SCRIPTS -->
  <script src="https://www.gstatic.com/firebasejs/7.20.0/firebase-app.js"></script>
  <!--<script src="https://www.gstatic.com/firebasejs/7.20.0/firebase-analytics.js"></script>-->
  <!--<script src="https://www.gstatic.com/firebasejs/7.20.0/firebase-auth.js"></script>-->
  <!--<script src="https://www.gstatic.com/firebasejs/7.20.0/firebase-firestore.js"></script>-->
  <!--<script src="https://www.gstatic.com/firebasejs/7.20.0/firebase-database.js"></script>-->
  <!--<script src="https://www.gstatic.com/firebasejs/7.20.0/firebase-storage.js"></script>-->
  <!--<script src="https://www.gstatic.com/firebasejs/7.20.0/firebase-messaging.js"></script>-->
  <!--<script src="https://www.gstatic.com/firebasejs/7.20.0/firebase-performance.js"></script>-->
  <script src="https://www.gstatic.com/firebasejs/7.20.0/firebase-remote-config.js"></script>
  <!--<script src="https://www.gstatic.com/firebasejs/7.20.0/firebase-functions.js"></script>-->
  <script>
      // Firebase configuration
      const firebaseConfig = {
        apiKey: "...",
        authDomain: "...",
        databaseURL: "...",
        projectId: "...",
        storageBucket: "...",
        messagingSenderId: "...",
        appId: "...",
        measurementId: "..."
      };
      // Initialize Firebase
      firebase.initializeApp(firebaseConfig);
  </script>
...

js_remote_config.dart

@JS()
library remote.config;

import 'dart:js_util' as js_util;

import 'package:js/js.dart';
import 'package:meta/meta.dart';

@JS('firebase.remoteConfig')
external _JSRemoteConfigInternal _getRemoteConfig();

@JS('Object.keys')
external List<String> _objectKeys(dynamic jsObject);

@JS()
@anonymous
class _JSRemoteConfigInternal {
  external _JSRemoteConfigInternal();

  @JS('defaultConfig')
  external set defaultConfig(Object defaults);

  @JS('defaultConfig')
  external Object get defaultConfig;

  @JS('fetchAndActivate')
  external Future<void> fetchAndActivate();

  @JS('getString')
  external String getString(String key);

  @JS('getNumber')
  external num getNumber(String key);

  @JS('getBoolean')
  external bool getBoolean(String key);
}

/// The entry point for accessing Remote Config.
@immutable
class JsRemoteConfig with _JSConverter {
  final _JSRemoteConfigInternal _internalDelegate;
  static JsRemoteConfig _instance;
  factory JsRemoteConfig() => _instance ??= JsRemoteConfig._internal();
  JsRemoteConfig._internal() : _internalDelegate = _getRemoteConfig();

  /// Sets the default config.
  ///
  /// Default config parameters should be set then when changes are needed the
  /// parameters should be updated in the Firebase console.
  set defaultConfig(Map<String, Object> defaults) => _internalDelegate.defaultConfig = _mapToJsObject(defaults);

  /// Gets the default config.
  Map<String, Object> get defaultConfig => _jsObjectToMap(_internalDelegate.defaultConfig);

  /// Activates the fetched config, makes fetched key-values take effect.
  Future<void> fetchAndActivate() => js_util.promiseToFuture<void>(_internalDelegate.fetchAndActivate());

  /// Gets the value corresponding to the [key] as a String.
  ///
  /// If there is no parameter with corresponding [key] then the default
  /// String value is returned.
  String getString(String key) => _internalDelegate.getString(key);

  /// Gets the value corresponding to the [key] as an number.
  ///
  /// If there is no parameter with corresponding [key] then the default
  /// number value is returned.
  num getNumber(String key) => _internalDelegate.getNumber(key);

  /// Gets the value corresponding to the [key] as an int.
  ///
  /// If there is no parameter with corresponding [key] then the default
  /// int value is returned.
  int getInt(String key) => getNumber(key).toInt();

  /// Gets the value corresponding to the [key] as a double.
  ///
  /// If there is no parameter with corresponding [key] then the default double
  /// value is returned.
  double getDouble(String key) => getNumber(key).toDouble();

  /// Gets the value corresponding to the [key] as a bool.
  ///
  /// If there is no parameter with corresponding [key] then the default bool
  /// value is returned.
  bool getBoolean(String key) => _internalDelegate.getBoolean(key);
}

mixin _JSConverter {
  Object _mapToJsObject(Map<String, Object> map) {
    final Object jsObject = js_util.newObject();
    for (final entry in map.entries) {
      js_util.setProperty(jsObject, entry.key, entry.value);
    }
    return jsObject;
  }

  Map<String, Object> _jsObjectToMap(Object jsObject) {
    final map = <String, Object>{};
    if (jsObject == null || jsObject is String || jsObject is num || jsObject is bool) return map;
    final keys = _objectKeys(jsObject);
    for (final key in keys) {
      map[key] = js_util.getProperty(jsObject, key);
    }
    return map;
  }
}

web_example.dart

final jsRemoteConfig = JsRemoteConfig();

jsRemoteConfig.defaultConfig = <String, Object>{
  'stringValue': byDefault.stringValue,
  'intValue': byDefault.intValue,
};

await jsRemoteConfig.fetchAndActivate().timeout(const Duration(seconds: 2));

final stringValue = jsRemoteConfig.getString('stringValue');
final intValue = jsRemoteConfig.getInt('intValue');

Awesome! Ready to create a PR.