Fix some strong mode issues.

lib/transformations/mixin_full_resolution.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 kernel.transformations.mixin_full_resolution;
 
 import '../ast.dart';
 import '../class_hierarchy.dart';
 import '../clone.dart';
 import '../core_types.dart';
 import '../type_algebra.dart';
@@ skipping 313 matching lines @@
 
     // Do we give named that we don't take?
     Set<String> givenNamed = arguments.named.map((v) => v.name).toSet();
     Set<String> takenNamed =
         targetFunction.namedParameters.map((v) => v.name).toSet();
     givenNamed.removeAll(takenNamed);
     return givenNamed.isEmpty;
   }
 }
 
-class SuperInitializerResolutionTransformer extends Transformer {
+class SuperInitializerResolutionTransformer extends InitializerVisitor {
   final Class lookupClass;
 
   SuperInitializerResolutionTransformer(this.lookupClass);
 
   transformInitializers(List<Initializer> initializers) {
     for (var initializer in initializers) {
       initializer.accept(this);
     }
   }
 
   visitSuperInitializer(SuperInitializer node) {
     Constructor constructor = node.target;
     if (constructor.enclosingClass != lookupClass) {
       // If [node] refers to a constructor target which is not directly the
       // superclass but some indirect base class then this is because classes in
       // the middle are mixin applications.  These mixin applications will have
       // received a forwarding constructor which we are required to use instead.
       for (var replacement in lookupClass.constructors) {
         if (constructor.name == replacement.name) {
           node.target = replacement;
-          return;
+          return null;
         }
       }
 
       throw new Exception(
           'Could not find a generative constructor named "${constructor.name}" '
           'in lookup class "${lookupClass.name}"!');
     }
   }
 }