null is an instance of Dynamic (fix issue 443).

runtime/vm/code_generator.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/code_generator.h"
 
 #include "vm/code_index_table.h"
 #include "vm/code_patcher.h"
 #include "vm/compiler.h"
 #include "vm/dart_api_impl.h"
@@ skipping 304 matching lines @@
 // Arg1: type.
 // Arg2: type arguments of the instantiator of the type.
 // Return value: true or false.
 DEFINE_RUNTIME_ENTRY(Instanceof, 3) {
   ASSERT(arguments.Count() == kInstanceofRuntimeEntry.argument_count());
   const Instance& instance = Instance::CheckedHandle(arguments.At(0));
   const Type& type = Type::CheckedHandle(arguments.At(1));
   const TypeArguments& type_instantiator =
       TypeArguments::CheckedHandle(arguments.At(2));
   ASSERT(type.IsFinalized());
-  ASSERT(!instance.IsNull());
   const Bool& result = Bool::Handle(
       instance.IsInstanceOf(type, type_instantiator) ?
       Bool::True() : Bool::False());
   if (FLAG_trace_type_checks) {
     const Type& instance_type = Type::Handle(instance.GetType());
     if (type.IsInstantiated()) {
       OS::Print("InstanceOf: '%s' %s '%s'\n",
                 String::Handle(instance_type.Name()).ToCString(),
                 (result.raw() == Bool::True()) ? "is" : "is !",
                 String::Handle(type.Name()).ToCString());
@@ skipping 677 matching lines @@
       }
     }
   }
   // The cache is null terminated, therefore the loop above should never
   // terminate by itself.
   UNREACHABLE();
   return Code::null();
 }
 
 }  // namespace dart