Provide an Expando implementation for the VM.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index af8695abcf43b802c1adeeeed8abc8553a5c65a3..2b02d0050bc7e8e5b59dc539d47ec5f6670ac2e4 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -624,12 +624,6 @@ RawError* Object::Init(Isolate* isolate) {
   RegisterClass(cls, name, core_impl_lib);
   pending_classes.Add(cls, Heap::kOld);
 
-  cls = Class::New<WeakProperty>();
-  object_store->set_weak_property_class(cls);
-  name = Symbols::WeakProperty();
-  RegisterClass(cls, name, core_impl_lib);
-  pending_classes.Add(cls, Heap::kOld);
-
   // Initialize the base interfaces used by the core VM classes.
   const Script& script = Script::Handle(Bootstrap::LoadCoreScript(false));
 
@@ -747,6 +741,11 @@ RawError* Object::Init(Isolate* isolate) {
   name = Symbols::_ExternalFloat64Array();
   RegisterPrivateClass(cls, name, core_lib);
 
+  cls = Class::New<WeakProperty>();
+  object_store->set_weak_property_class(cls);
+  name = Symbols::WeakProperty();
+  RegisterClass(cls, name, core_lib);
+
   // Set the super type of class Stacktrace to Object type so that the
   // 'toString' method is implemented.
   cls = object_store->stacktrace_class();