Use VM type cast and save handles.

runtime/lib/isolate.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 "platform/assert.h"
 #include "vm/bootstrap_natives.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
@@ skipping 308 matching lines @@
                         error,
                         library_url,
                         class_name);
   }
 
   // TODO(turnidge): Move this code up before we launch the new
   // thread.  That way we won't have a thread hanging around that we
   // can't talk to.
   const Object& port = Object::Handle(DartLibraryCalls::NewSendPort(port_id));
   if (port.IsError()) {
-    Exceptions::PropagateError(port);
+    Exceptions::PropagateError(Error::Cast(port));
   }
   arguments->SetReturn(port);
 }
 
 
 DEFINE_NATIVE_ENTRY(ReceivePortImpl_factory, 1) {
   ASSERT(AbstractTypeArguments::CheckedHandle(arguments->At(0)).IsNull());
   intptr_t port_id =
       PortMap::CreatePort(arguments->isolate()->message_handler());
   const Object& port = Object::Handle(ReceivePortCreate(port_id));
   if (port.IsError()) {
-    Exceptions::PropagateError(port);
+    Exceptions::PropagateError(Error::Cast(port));
   }
   arguments->SetReturn(port);
 }
 
 
 DEFINE_NATIVE_ENTRY(ReceivePortImpl_closeInternal, 1) {
   GET_NATIVE_ARGUMENT(Smi, id, arguments->At(0));
   PortMap::ClosePort(id.Value());
 }
 
@@ skipping 234 matching lines @@
     free(error);
     ThrowIsolateSpawnException(msg);
   }
 
   // Try to create a SendPort for the new isolate.
   const Object& port = Object::Handle(
       DartLibraryCalls::NewSendPort(state->isolate()->main_port()));
   if (port.IsError()) {
     state->Cleanup();
     delete state;
-    Exceptions::PropagateError(port);
+    Exceptions::PropagateError(Error::Cast(port));
   }
 
   // Start the new isolate.
   state->isolate()->set_spawn_data(reinterpret_cast<uword>(state));
   state->isolate()->message_handler()->Run(
       Dart::thread_pool(), RunIsolate2, ShutdownIsolate,
       reinterpret_cast<uword>(state->isolate()));
 
   arguments->SetReturn(port);
 }
@@ skipping 34 matching lines @@
     ThrowIsolateSpawnException(msg);
   }
 
   Spawn(arguments, new SpawnState(canonical_uri));
 }
 
 
 DEFINE_NATIVE_ENTRY(isolate_getPortInternal, 0) {
   const Object& port = Object::Handle(ReceivePortCreate(isolate->main_port()));
   if (port.IsError()) {
-    Exceptions::PropagateError(port);
+    Exceptions::PropagateError(Error::Cast(port));
   }
   arguments->SetReturn(port);
 }
 
 }  // namespace dart