Load App.framework in macOS app (flutter#23221)

Author: jmagman

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

@@ -32,6 +32,9 @@ - (instancetype)initWithPrecompiledDartBundle:(NSBundle*)bundle {
     _dartBundle = [NSBundle bundleWithURL:[NSBundle.mainBundle.privateFrameworksURL
                                               URLByAppendingPathComponent:@"App.framework"]];
   }
+  if (!_dartBundle.isLoaded) {
+    [_dartBundle load];
+  }
   _dartEntrypointArguments = [[NSProcessInfo processInfo] arguments];
   // Remove the first element as it's the binary name
   _dartEntrypointArguments = [_dartEntrypointArguments