Load the native call wrapper via Thread.

runtime/vm/native_entry.h

Index: runtime/vm/native_entry.h
diff --git a/runtime/vm/native_entry.h b/runtime/vm/native_entry.h
index 4dd655d24d4aa7624453a1f6ec0d9898da28be0e..0123b47383ca9869c4bf7cb75243627f50d56345 100644
--- a/runtime/vm/native_entry.h
+++ b/runtime/vm/native_entry.h
@@ -114,11 +114,11 @@ class NativeEntry : public AllStatic {
                                                uword pc);
   static const uint8_t* ResolveSymbol(uword pc);
 
-  static const ExternalLabel& NativeCallWrapperLabel();
+  static uword NativeCallWrapperEntry();
   static void NativeCallWrapper(Dart_NativeArguments args,
                                 Dart_NativeFunction func);
 
-  static const ExternalLabel& LinkNativeCallLabel();
+  static uword LinkNativeCallEntry();
   static void LinkNativeCall(Dart_NativeArguments args);
 };