Dart fix + Flutter upgrade

lib/app_sources/apkpure.dart

@@ -8,7 +8,7 @@ import 'package:obtainium/providers/source_provider.dart';
 
 extension Unique<E, Id> on List<E> {
   List<E> unique([Id Function(E element)? id, bool inplace = true]) {
-    final ids = Set();
+    final ids = <dynamic>{};
     var list = inplace ? this : List<E>.from(this);
     list.retainWhere((x) => ids.add(id != null ? id(x) : x as Id));
     return list;
@@ -76,7 +76,7 @@ class APKPure extends AppSource {
     return Uri.parse(standardUrl).pathSegments.last;
   }
 
-  getDetailsForVersion(
+  Future<APKDetails> getDetailsForVersion(
     List<Map<String, dynamic>> versionVariants,
     List<String> supportedArchs,
     Map<String, dynamic> additionalSettings,

lib/app_sources/directAPKLink.dart

@@ -16,7 +16,7 @@ class DirectAPKLink extends AppSource {
                 .where((element) => element.key == 'requestHeader')
                 .isNotEmpty,
           )
-          .toList(),
+          ,
       [
         GeneratedFormDropdown(
           'defaultPseudoVersioningMethod',

lib/app_sources/github.dart

@@ -566,7 +566,7 @@ class GitHub extends AppSource {
     }
   }
 
-  getLatestAPKDetailsCommon2(
+  Future<APKDetails> getLatestAPKDetailsCommon2(
     String standardUrl,
     Map<String, dynamic> additionalSettings,
     Future<String> Function(bool) reqUrlGenerator,
@@ -667,7 +667,7 @@ class GitHub extends AppSource {
     );
   }
 
-  rateLimitErrorCheck(Response res) {
+  void rateLimitErrorCheck(Response res) {
     if (res.headers['x-ratelimit-remaining'] == '0') {
       throw RateLimitError(
         (int.parse(res.headers['x-ratelimit-reset'] ?? '1800000000') / 60000000)

lib/app_sources/huaweiappgallery.dart

@@ -24,10 +24,10 @@ class HuaweiAppGallery extends AppSource {
     return match.group(0)!;
   }
 
-  getDlUrl(String standardUrl) =>
+  String getDlUrl(String standardUrl) =>
       'https://${hosts[0].replaceAll('appgallery.huawei', 'appgallery.cloud.huawei')}/appdl/${standardUrl.split('/').last}';
 
-  requestAppdlRedirect(
+  Future<Response> requestAppdlRedirect(
     String dlUrl,
     Map<String, dynamic> additionalSettings,
   ) async {
@@ -45,7 +45,7 @@ class HuaweiAppGallery extends AppSource {
     }
   }
 
-  appIdFromRedirectDlUrl(String redirectDlUrl) {
+  String appIdFromRedirectDlUrl(String redirectDlUrl) {
     var parts = redirectDlUrl
         .split('?')[0]
         .split('/')

lib/app_sources/neutroncode.dart

@@ -56,7 +56,7 @@ class NeutronCode extends AppSource {
     }
   }
 
-  customDateParse(String dateString) {
+  String? customDateParse(String dateString) {
     List<String> parts = dateString.split(' ');
     if (parts.length != 3) {
       return null;

lib/app_sources/uptodown.dart

@@ -3,7 +3,7 @@ import 'package:html/parser.dart';
 import 'package:obtainium/custom_errors.dart';
 import 'package:obtainium/providers/source_provider.dart';
 
-parseDateTimeMMMddCommayyyy(String? dateString) {
+DateTime? parseDateTimeMMMddCommayyyy(String? dateString) {
   DateTime? releaseDate;
   try {
     releaseDate = dateString != null