scriptSource property Null safety

Implementation

Future<String> get scriptSource async {
  final typeSource = (await _getClass(executableType)).toSource();
  final builder = StringBuffer();

  builder.writeln("import 'dart:async';");
  builder.writeln("import 'dart:isolate';");
  builder.writeln("import 'dart:mirrors';");
  for (final anImport in imports) {
    builder.writeln("import '$anImport';");
  }
  builder.writeln(
    """
Future main (List<String> args, Map<String, dynamic> message) async {
final sendPort = message['_sendPort'];
final executable = $typeName(message);
final result = await executable.execute();
sendPort.send({"_result": result});
}
  """,
  );
  builder.writeln(typeSource);

  builder.writeln((await _getClass(Executable)).toSource());
  for (final type in additionalTypes) {
    final source = await _getClass(type);
    builder.writeln(source.toSource());
  }

  if (additionalContents != null) {
    builder.writeln(additionalContents);
  }

  return builder.toString();
}