Initial UserTag and dart:profiler library

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 c4f063e0a470a3b16ffdde20761c3f8c6ee46eb5..08e0ecb2e5a9a3659e85a2d1d7782e806f1d9dbc 100644
--- a/runtime/vm/raw_object_snapshot.cc
+++ b/runtime/vm/raw_object_snapshot.cc
@@ -2693,4 +2693,27 @@ void RawMirrorReference::WriteTo(SnapshotWriter* writer,
   }
 }
 
+
+RawUserTag* UserTag::ReadFrom(SnapshotReader* reader,
+                              intptr_t object_id,
+                              intptr_t tags,
+                              Snapshot::Kind kind) {
+  UNREACHABLE();
+  return UserTag::null();
+}
+
+
+void RawUserTag::WriteTo(SnapshotWriter* writer,
+                         intptr_t object_id,
+                         Snapshot::Kind kind) {
+  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 UserTag)");
+  } else {
+    UNREACHABLE();
+  }
+}
+
 }  // namespace dart