Raise an exception instead of crashing when attempting to pass a MirrorReference between isolates.

runtime/vm/raw_object_snapshot.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/bigint_operations.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/snapshot.h"
 #include "vm/symbols.h"
 #include "vm/visitor.h"
@@ skipping 2636 matching lines @@
                                           intptr_t tags,
                                           Snapshot::Kind kind) {
   UNREACHABLE();
   return MirrorReference::null();
 }
 
 
 void RawMirrorReference::WriteTo(SnapshotWriter* writer,
                                  intptr_t object_id,
                                  Snapshot::Kind kind) {
-  UNREACHABLE();
+  if (kind == Snapshot::kMessage) {
+    // We do not allow objects with native fields in an isolate message.
+    writer->SetWriteException(Exceptions::kArgument,
+                              "Illegal argument in isolate message"
+                              " : (object is a MirrorReference)");
+  } else {
+    UNREACHABLE();
+  }
 }
 
 }  // namespace dart