Use the return value of vm native methods to set the return value,

lib/object.cc

Index: lib/object.cc
===================================================================
--- lib/object.cc	(revision 11528)
+++ lib/object.cc	(working copy)
@@ -13,7 +13,7 @@
 DEFINE_NATIVE_ENTRY(Object_toString, 1) {
   const Instance& instance = Instance::CheckedHandle(arguments->At(0));
   const char* c_str = instance.ToCString();
-  arguments->SetReturn(String::Handle(String::New(c_str)));
+  return String::New(c_str);
 }
 
 
@@ -53,6 +53,7 @@
     dart_arguments.Add(&array);
   }
   Exceptions::ThrowByType(Exceptions::kNoSuchMethod, dart_arguments);
+  return Object::null();
 }
 
 }  // namespace dart