[dart-gen] Fix lists containing atomic types

[fondoger]

Before

Before will, this code will throw an error:
Unhandled Exception: type 'List<dynamic>' is not a subtype of type 'List<String>'.

  factory SampleResponse.fromJson(Map<String, dynamic> m) {
    return SampleResponse(
      type: m['type'] ?? "",
      imageUrls: m['imageUrls'] ?? [],               ### Code can compile, but it will throw an error during run-time.
    );
  }

After

After fix, this code runs smoothly.

  factory SampleResponse.fromJson(Map<String, dynamic> m) {
    return SampleResponse(
      type: m['type'] ?? "",
      imageUrls: m['imageUrls']?.cast<String>() ?? [],
    );
  }