Fix serialization of forwarding constructors.

tests/compiler/dart2js/serialization_helper.dart

Index: tests/compiler/dart2js/serialization_helper.dart
diff --git a/tests/compiler/dart2js/serialization_helper.dart b/tests/compiler/dart2js/serialization_helper.dart
index 43b2217e0ab7e2331c5c60484b003b55a9510d62..b261f74fca81b16f9f05769472f1fd3d9b358614 100644
--- a/tests/compiler/dart2js/serialization_helper.dart
+++ b/tests/compiler/dart2js/serialization_helper.dart
@@ -232,12 +232,9 @@ class _DeserializerSystem extends DeserializerSystem {
   @override
   WorldImpact computeWorldImpact(Element element) {
     ResolutionImpact resolutionImpact = getResolutionImpact(element);
-    if (resolutionImpact == null) {
-      print('No impact found for $element (${element.library})');
-      return const WorldImpact();
-    } else {
-      return _impactTransformer.transformResolutionImpact(resolutionImpact);
-    }
+    assert(invariant(element, resolutionImpact != null,
+        message: 'No impact found for $element (${element.library})'));
+    return _impactTransformer.transformResolutionImpact(resolutionImpact);
   }
 
   @override