Support Null type in Class::TypeTest and simplify Instance::IsInstanceOf.

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 "vm/exceptions.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
 #include "vm/stack_frame.h"
@@ skipping 43 matching lines @@
     }
   }
   Exceptions::ThrowByType(Exceptions::kNoSuchMethod, dart_arguments);
   return Object::null();
 }
 
 
 DEFINE_NATIVE_ENTRY(Object_runtimeType, 1) {
   const Instance& instance = Instance::CheckedHandle(arguments->NativeArgAt(0));
   const Type& type = Type::Handle(instance.GetType());
+  // The static type of null is specified to be the bottom type, however, the
+  // runtime type of null is the Null type, which we correctly return here.
   return type.Canonicalize();
 }
 
 
-
 DEFINE_NATIVE_ENTRY(Object_instanceOf, 5) {
   const Instance& instance = Instance::CheckedHandle(arguments->NativeArgAt(0));
   // Instantiator at position 1 is not used. It is passed along so that the call
   // can be easily converted to an optimized implementation. Instantiator is
   // used to populate the subtype cache.
   const AbstractTypeArguments& instantiator_type_arguments =
       AbstractTypeArguments::CheckedHandle(arguments->NativeArgAt(2));
   const AbstractType& type =
       AbstractType::CheckedHandle(arguments->NativeArgAt(3));
   const Bool& negate = Bool::CheckedHandle(arguments->NativeArgAt(4));
@@ skipping 19 matching lines @@
 }
 
 
 DEFINE_NATIVE_ENTRY(AbstractType_toString, 1) {
   const AbstractType& type =
       AbstractType::CheckedHandle(arguments->NativeArgAt(0));
   return type.UserVisibleName();
 }
 
 }  // namespace dart