fix: use stub file for dart:io usage

Author: Vinzent03

packages/supabase/lib/src/constants.dart

@@ -1,17 +1,7 @@
 import 'package:supabase/src/version.dart';
-import 'dart:io' show Platform;
-
-const bool kIsWeb = bool.fromEnvironment('dart.library.js_util');
+import 'platform_stub.dart' if (dart.library.io) 'platform_io.dart';
 
 class Constants {
-  static String? get platform {
-    return kIsWeb ? null : Platform.operatingSystem;
-  }
-
-  static String? get platformVersion {
-    return kIsWeb ? null : Platform.operatingSystemVersion;
-  }
-
   static final Map<String, String> defaultHeaders = {
     'X-Client-Info': 'supabase-dart/$version',
     if (platform != null)

packages/supabase/lib/src/platform_io.dart

@@ -0,0 +1,9 @@
+import 'dart:io';
+
+String? get platform {
+  return Platform.operatingSystem;
+}
+
+String? get platformVersion {
+  return Platform.operatingSystemVersion;
+}

packages/supabase/lib/src/platform_stub.dart

@@ -0,0 +1,7 @@
+String? get platform {
+  return null;
+}
+
+String? get platformVersion {
+  return null;
+}