Use the main bundle if the App bundle is not found (flutter#18749)

It looks like half the code in this function was updated to use the
main bundle but this one line was still only using the _dartBundle. I am
pretty sure that is a mistake and instead we want to use the mainBundle
as a backup both to find the plist and to find the assets.

shell/platform/darwin/macos/framework/Source/FlutterDartProject.mm

@@ -47,7 +47,7 @@ - (NSString*)assetsPath {
   if (flutterAssetsName == nil) {
     flutterAssetsName = @"flutter_assets";
   }
-  NSString* path = [_dartBundle pathForResource:flutterAssetsName ofType:@""];
+  NSString* path = [assetBundle pathForResource:flutterAssetsName ofType:@""];
   if (!path) {
     NSLog(@"Failed to find path for \"%@\"", flutterAssetsName);
   }