Add a VM defined class MirrorReference as an opaque pointer for Dart code to VM internal objects.

runtime/vm/raw_object_snapshot.cc

Index: runtime/vm/raw_object_snapshot.cc
===================================================================
--- runtime/vm/raw_object_snapshot.cc	(revision 24693)
+++ runtime/vm/raw_object_snapshot.cc	(working copy)
@@ -2646,4 +2646,20 @@
   writer->Write<RawObject*>(ptr()->value_);
 }
 
+
+RawMirrorReference* MirrorReference::ReadFrom(SnapshotReader* reader,
+                                          intptr_t object_id,
+                                          intptr_t tags,
+                                          Snapshot::Kind kind) {
+  UNREACHABLE();
+  return MirrorReference::null();
+}
+
+
+void RawMirrorReference::WriteTo(SnapshotWriter* writer,
+                                 intptr_t object_id,
+                                 Snapshot::Kind kind) {
+  UNREACHABLE();
+}
+
 }  // namespace dart