Add a weak property type to the virtual machine.

runtime/vm/object_store.cc

Index: runtime/vm/object_store.cc
diff --git a/runtime/vm/object_store.cc b/runtime/vm/object_store.cc
index 928dbc4306501544cb51babfbaa1a582b44ab3e6..c389172052cee85fa5b568fe6c156b0f46f16f0a 100644
--- a/runtime/vm/object_store.cc
+++ b/runtime/vm/object_store.cc
@@ -59,6 +59,7 @@ ObjectStore::ObjectStore()
     external_float64_array_class_(Class::null()),
     stacktrace_class_(Class::null()),
     jsregexp_class_(Class::null()),
+    weak_property_class_(Class::null()),
     true_value_(Bool::null()),
     false_value_(Bool::null()),
     empty_array_(Array::null()),
@@ -177,6 +178,7 @@ RawClass* ObjectStore::GetClass(int index) {
     case kExternalFloat64ArrayClass: return external_float64_array_class_;
     case kStacktraceClass: return stacktrace_class_;
     case kJSRegExpClass: return jsregexp_class_;
+    case kWeakPropertyClass: return weak_property_class_;
     default: break;
   }
   UNREACHABLE();