Don't allow for creation of CallbackHandle objects for anonymous closures (#6864)

* Don't allow for creation of CallbackHandle objects for anonymous closures. Fixes issue #22310

lib/ui/dart_runtime_hooks.cc

@@ -318,12 +318,14 @@ static std::string GetFunctionName(Dart_Handle func) {
 }
 
 void GetCallbackHandle(Dart_NativeArguments args) {
+  const char* kAnonymousClosureName = "<anonymous closure>";
   Dart_Handle func = Dart_GetNativeArgument(args, 0);
   std::string name = GetFunctionName(func);
   std::string class_name = GetFunctionClassName(func);
   std::string library_path = GetFunctionLibraryUrl(func);
 
-  if (name.empty()) {
+  // TODO(24394): check !Dart_IsTearOff(func) instead of string comparison.
+  if (name.empty() || (name == kAnonymousClosureName)) {
     Dart_SetReturnValue(args, Dart_Null());
     return;
   }