fix: improve error handling in _firebaseExceptionFromCoreFirebaseError

Author: SelaseKay

packages/_flutterfire_internals/lib/_flutterfire_internals.dart

@@ -91,12 +91,14 @@ FirebaseException _firebaseExceptionFromCoreFirebaseError(
 ///
 /// See also https://github.com/dart-lang/sdk/issues/30741
 bool _testException(Object? objectException) {
-  final exception = objectException! as core_interop.JSError;
+  if (objectException is! core_interop.JSError) {
+    return false;
+  }
 
-  final message = _safeConvertFromPossibleJSObject(exception.message);
+  final message = _safeConvertFromPossibleJSObject(objectException.message);
   // Firestore web does not contain `Firebase` in the message so we check the exception itself.
   return message.contains('Firebase') ||
-      exception.toString().contains('FirebaseError');
+      objectException.toString().contains('FirebaseError');
 }
 
 /// Transforms internal errors in something more readable for end-users.

packages/_flutterfire_internals/test/guard_test.dart

@@ -52,6 +52,19 @@ void main() {
         expect(stack, current);
       }
     });
+
+    test(
+        'propagates plain Dart errors from Futures (e.g. ArgumentError on web)',
+        () async {
+      await expectLater(
+        guardWebExceptions(
+          () => Future<void>.error(ArgumentError('test')),
+          plugin: 'test',
+          codeParser: (c) => c,
+        ),
+        throwsA(isA<ArgumentError>()),
+      );
+    });
   });
 }