Use patching for Expando so the implementation-specific files are not needed in the SDK.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index ca07985df0bcc4a5af41cf559d1332db10185d7a..3f8cae887acce57d73cdafaadcba2556583f5854 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -629,6 +629,12 @@ 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();
+  RegisterPrivateClass(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));
 
@@ -746,11 +752,6 @@ 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();
-  RegisterPrivateClass(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();