Fix serialization of forwarding constructors.

tests/compiler/dart2js/serialization_helper.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.serialization_helper;
 
 import 'dart:io';
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
@@ skipping 214 matching lines @@
   }
 
   @override
   ResolutionImpact getResolutionImpact(Element element) {
     return _resolutionImpactDeserializer.impactMap[element];
   }
 
   @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
   bool isDeserialized(Element element) {
     return deserializedLibraries.contains(element.library);
   }
 }
 
 const String RESOLVED_AST_TAG = 'resolvedAst';
 
@@ skipping 59 matching lines @@
 
   @override
   void onElement(Element element, ObjectDecoder getDecoder(String tag)) {
     ObjectDecoder decoder = getDecoder(RESOLVED_AST_TAG);
     if (decoder != null) {
       _decoderMap[element] = decoder;
     }
   }
 }