Add a weak property type to the virtual machine.

runtime/vm/class_finalizer.cc

Index: runtime/vm/class_finalizer.cc
diff --git a/runtime/vm/class_finalizer.cc b/runtime/vm/class_finalizer.cc
index df5c5acd265e5c054b622f859b73584ff1fd5692..b0c1a973902718d9f7c546535fd9b5d32ded3aef 100644
--- a/runtime/vm/class_finalizer.cc
+++ b/runtime/vm/class_finalizer.cc
@@ -263,6 +263,8 @@ void ClassFinalizer::VerifyBootstrapClasses() {
   ASSERT(ExternalFloat32Array::InstanceSize() == cls.instance_size());
   cls = object_store->external_float64_array_class();
   ASSERT(ExternalFloat64Array::InstanceSize() == cls.instance_size());
+  cls = object_store->weak_property_class();
+  ASSERT(WeakProperty::InstanceSize() == cls.instance_size());
 #endif  // defined(DEBUG)
 
   // Remember the currently pending classes.
@@ -374,7 +376,8 @@ void ClassFinalizer::ResolveSuperType(const Class& cls) {
       case kFloat32Array:
       case kExternalFloat32Array:
       case kFloat64Array:
-      case kExternalFloat64Array: {
+      case kExternalFloat64Array:
+      case kWeakProperty: {
         const Script& script = Script::Handle(cls.script());
         ReportError(script, cls.token_pos(),
                     "'%s' is not allowed to extend '%s'",