[BUG][DART2] ApiClient deserialize result to type Map<String,Map<String,String>> throws exception

my swagger has return type of "Map<String,Map<String,String>>"
when using dart2 generate clientapi ,
it use such code to convert the response to Map<String,Map<String,String>>, but throw exception

api.mustache : line 182:

`return {{{returnType}}}.from(await apiClient.deserializeAsync(await _decodeBodyBytes(response), '{{{returnType}}}'),);

the exception is :
type '_Map<String, dynamic>' is not a subtype of type 'Map<String, String>' in type cast

here is minimal code to reproduce this exception :

void main() async {
  final client = ApiClient();
  var networkResponseMap = await client.deserializeAsync('{"k1":{ "k11":"v11","k12":"v12"},"k2":{ "k21":"v21","k22":"v22"}}', 'Map<String,Map<String,String>>');
  var apiResult =  Map<String, Map<String, String >>.from(networkResponseMap);
  print(apiResult);
}

`

Start with api.mustache at line 182 and the ApiClient.deserializeAsync call shown in the issue. Run the minimal Dart reproduction using the nested Map<String,Map<String,String>> response, then inspect the generated conversion path. Done means the nested map response deserializes without the reported type-cast exception.