Fix some strong mode issues.

lib/transformations/mixin_full_resolution.dart

Index: lib/transformations/mixin_full_resolution.dart
diff --git a/lib/transformations/mixin_full_resolution.dart b/lib/transformations/mixin_full_resolution.dart
index dfcc3c8174690aac765a9fd16a5c524a4bc50e48..b08606218efe8c7b350a0624c7c78fec346fe5f0 100644
--- a/lib/transformations/mixin_full_resolution.dart
+++ b/lib/transformations/mixin_full_resolution.dart
@@ -331,7 +331,7 @@ class SuperCallResolutionTransformer extends Transformer {
   }
 }
 
-class SuperInitializerResolutionTransformer extends Transformer {
+class SuperInitializerResolutionTransformer extends InitializerVisitor {
   final Class lookupClass;
 
   SuperInitializerResolutionTransformer(this.lookupClass);
@@ -352,7 +352,7 @@ class SuperInitializerResolutionTransformer extends Transformer {
       for (var replacement in lookupClass.constructors) {
         if (constructor.name == replacement.name) {
           node.target = replacement;
-          return;
+          return null;
         }
       }