Transpile value types (int, double, bool, etc...) as nullable in C#

[JeroMiya]

All types are nullable in the current version of Dart (though in the future this will not be the case), but value types are not by default in C#. A number of syntax errors in the transpiler output are a result of this difference. For now, let's output all of these types as nullable by default until the analyzer can tell us whether or not any given instance is nullable or not:
https://dart.dev/null-safety