Move Null type to the Bottom in the VM (fixes #28025).

runtime/lib/object.cc

 // Copyright (c) 2011, 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.
 
 #include "vm/bootstrap_natives.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/code_patcher.h"
 #include "vm/exceptions.h"
 #include "vm/heap.h"
@@ skipping 293 matching lines @@
   const Instance& instance =
       Instance::CheckedHandle(zone, arguments->NativeArgAt(0));
   const TypeArguments& instantiator_type_arguments =
       TypeArguments::CheckedHandle(zone, arguments->NativeArgAt(1));
   AbstractType& type =
       AbstractType::CheckedHandle(zone, arguments->NativeArgAt(2));
   ASSERT(type.IsFinalized());
   ASSERT(!type.IsMalformed());
   ASSERT(!type.IsMalbounded());
   Error& bound_error = Error::Handle(zone);
-  if (instance.IsNull()) {
-    return instance.raw();
-  }
   const bool is_instance_of =
+      instance.IsNull() ||
       instance.IsInstanceOf(type, instantiator_type_arguments, &bound_error);
   if (FLAG_trace_type_checks) {
     const char* result_str = is_instance_of ? "true" : "false";
     OS::Print("Object.as: result %s\n", result_str);
     const AbstractType& instance_type =
         AbstractType::Handle(zone, instance.GetType(Heap::kNew));
     OS::Print("  instance type: %s\n",
               String::Handle(zone, instance_type.Name()).ToCString());
     OS::Print("  cast type: %s\n",
               String::Handle(zone, type.Name()).ToCString());
@@ skipping 76 matching lines @@
 
 DEFINE_NATIVE_ENTRY(Internal_inquireIs64Bit, 0) {
 #if defined(ARCH_IS_64_BIT)
   return Bool::True().raw();
 #else
   return Bool::False().raw();
 #endif  // defined(ARCH_IS_64_BIT)
 }
 
 }  // namespace dart