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

runtime/vm/raw_object_snapshot.cc

Index: runtime/vm/raw_object_snapshot.cc
diff --git a/runtime/vm/raw_object_snapshot.cc b/runtime/vm/raw_object_snapshot.cc
index e714f449257bcf527a5473efb9ac2da16ab4c41c..27ad524978ff13f4f4d9fcf8d8afb34ab0ad70b8 100644
--- a/runtime/vm/raw_object_snapshot.cc
+++ b/runtime/vm/raw_object_snapshot.cc
@@ -2654,7 +2654,14 @@ RawMirrorReference* MirrorReference::ReadFrom(SnapshotReader* reader,
 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