fix: add assertions for sealed classes

Author: O-Hannonen

json_serializable/lib/src/decode_helper.dart

@@ -159,7 +159,10 @@ mixin DecodeHelper implements HelperCore {
   /// '''
   /// ```
   String _createSealedFunctionExpressionBody() {
+    assert(element.isSealed);
+
     final implementations = sealedClassImplementations(element);
+
     final discriminator = config.unionDiscriminator;
 
     String buildSingleImpl(ClassElement impl) {

json_serializable/lib/src/encoder_helper.dart

@@ -139,6 +139,8 @@ mixin EncodeHelper implements HelperCore {
   /// '''
   /// ```
   String _createSealedFunctionExpressionBody() {
+    assert(element.isSealed);
+
     final implementations = sealedClassImplementations(element);
 
     final discriminator = config.unionDiscriminator;