Update Flutter sample to align closer with downstream DartPad version

[parlough]

• Allow ChatWidget to accept an API key, since we use a different method of specifying it, and users might want to as well.
• Follow member ordering and variable style consistently between classes
• Reduce max width of message slightly
• Avoid reevaluating _chat.history iterable then running toList for each item in ListView.builder

[devoncarew]

@johnpryan - can you review this? (cc @natebosch for FYI)

[lrhn]

You could make this nullable, instead of the empty string, as:

const String? _apiKey = bool.hasEnvironment('API_KEY') ? String.fromEnvironment('API_KEY') : null;`

or

const String _envApiKey = String.fromEnvironment('API_KEY');
const String? _apiKey = _envApiKey == "" ? null : _envApiKey;

[lrhn]

Consider providing the API key as a command line argument instead.

onst String _envApiKey = String.fromEnvironment('API_KEY');
final String? _apiKey = _envApiKey == "" ? null : _envApiKey;
void main(List<String> args) {
  String? apiKey = _apiKey;
  if (args.isNotEmpty) {
    apiKey = args.first;
  }
  if (apiKey == null) {
    stderr.writeln("No API key specified");
    exit(1);
  }
  // use it