Use VM type cast and save handles.

runtime/lib/isolate.cc

Index: runtime/lib/isolate.cc
===================================================================
--- runtime/lib/isolate.cc	(revision 9315)
+++ runtime/lib/isolate.cc	(working copy)
@@ -326,7 +326,7 @@
   // 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);
 }
@@ -338,7 +338,7 @@
       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);
 }
@@ -593,7 +593,7 @@
   if (port.IsError()) {
     state->Cleanup();
     delete state;
-    Exceptions::PropagateError(port);
+    Exceptions::PropagateError(Error::Cast(port));
   }
 
   // Start the new isolate.
@@ -648,7 +648,7 @@
 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);
 }