Use VM type cast and save handles.

runtime/lib/mirrors.cc

 // Copyright (c) 2012, 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 "platform/json.h"
 #include "include/dart_api.h"
 #include "include/dart_debugger_api.h"
 #include "vm/dart_entry.h"
 #include "vm/exceptions.h"
 #include "vm/message.h"
 #include "vm/port.h"
 #include "vm/resolver.h"
 
 namespace dart {
 
 DEFINE_NATIVE_ENTRY(Mirrors_isLocalPort, 1) {
   GET_NATIVE_ARGUMENT(Instance, port, arguments->At(0));
 
   // Get the port id from the SendPort instance.
   const Object& id_obj = Object::Handle(DartLibraryCalls::PortGetId(port));
   if (id_obj.IsError()) {
-    Exceptions::PropagateError(id_obj);
+    Exceptions::PropagateError(Error::Cast(id_obj));
     UNREACHABLE();
   }
   ASSERT(id_obj.IsSmi() || id_obj.IsMint());
   Integer& id = Integer::Handle();
   id ^= id_obj.raw();
   Dart_Port port_id = static_cast<Dart_Port>(id.AsInt64Value());
   const Bool& is_local = Bool::Handle(Bool::Get(PortMap::IsLocalPort(port_id)));
   arguments->SetReturn(is_local);
 }
 
@@ skipping 507 matching lines @@
   Dart_SetReturnValue(args, wrapped_result);
 }
 
 void HandleMirrorsMessage(Isolate* isolate,
                           Dart_Port reply_port,
                           const Instance& message) {
   UNIMPLEMENTED();
 }
 
 }  // namespace dart