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 5f010104dad49761fe6b2d949909b1d3987813f8..177f505fa6c8faa521c56eb75d52c9603ef7dbcc 100644
--- a/runtime/vm/raw_object.cc
+++ b/runtime/vm/raw_object.cc
@@ -971,4 +971,13 @@ 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