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

runtime/lib/object.cc

Index: runtime/lib/object.cc
===================================================================
--- runtime/lib/object.cc	(revision 16663)
+++ runtime/lib/object.cc	(working copy)
@@ -61,11 +61,12 @@
 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