Generated injectors fail on raw strings

[matanlurey]

(From internal issue b/112890636)

If you write the following:

import 'package:angular/angular.dart';
import 'this_file.template.dart' as ng;

@GenerateInjector(const [
  ValueProvider(String, r'$5.00 USD'),
])
final InjectorFactory example = ng.example$Injector;

... a compile-time error will be eventually issued, because we will generate:

'$5.00 USD' instead of r'$5.00 USD'. We need to retain the raw-ness of the String.