Add a weak property type to the virtual machine.

runtime/vm/object_store.h

Index: runtime/vm/object_store.h
diff --git a/runtime/vm/object_store.h b/runtime/vm/object_store.h
index 2c55926822771cd865069fa130bfb22eeedbcd0a..7747284391215d016c3202ef1c0a6dc7aa43164c 100644
--- a/runtime/vm/object_store.h
+++ b/runtime/vm/object_store.h
@@ -334,6 +334,13 @@ class ObjectStore {
     return OFFSET_OF(ObjectStore, jsregexp_class_);
   }
 
+  RawClass* weak_property_class() const {
+    return weak_property_class_;
+  }
+  void set_weak_property_class(const Class& value) {
+    weak_property_class_ = value.raw();
+  }
+
   RawArray* symbol_table() const { return symbol_table_; }
   void set_symbol_table(const Array& value) { symbol_table_ = value.raw(); }
 
@@ -506,6 +513,7 @@ class ObjectStore {
   RawClass* external_float64_array_class_;
   RawClass* stacktrace_class_;
   RawClass* jsregexp_class_;
+  RawClass* weak_property_class_;
   RawBool* true_value_;
   RawBool* false_value_;
   RawArray* symbol_table_;