Add a weak property type to the virtual machine.

runtime/vm/raw_object.cc

Index: runtime/vm/raw_object.cc
diff --git a/runtime/vm/raw_object.cc b/runtime/vm/raw_object.cc
index a084f7ca9ee9c9bdcda13b83e76e9990700c2f25..b4fd809901db10094447050c20ed1393eebd8456 100644
--- a/runtime/vm/raw_object.cc
+++ b/runtime/vm/raw_object.cc
@@ -960,4 +960,13 @@ intptr_t RawJSRegExp::VisitJSRegExpPointers(RawJSRegExp* raw_obj,
   return JSRegExp::InstanceSize(length);
 }
 
+
+intptr_t RawWeakProperty::VisitWeakPropertyPointers(
+    RawWeakProperty* raw_obj, ObjectPointerVisitor* visitor) {
+  // Make sure that we got here with the tagged pointer as this.
+  ASSERT(raw_obj->IsHeapObject());
+  visitor->VisitPointers(raw_obj->from(), raw_obj->to());
+  return WeakProperty::InstanceSize();
+}
+
 }  // namespace dart