anchor.diff:  import "package:front_end/src/api_prototype/memory_file_system.dart"
       show MemoryFileSystem;
   
  +import 'package:front_end/src/fasta/fasta_codes.dart' show LocatedMessage;
  +
  +import 'package:front_end/src/fasta/severity.dart' show Severity;
  +
   Directory outDir;
   
   main() async {
  
positive.code:   } finally {
      outDir.deleteSync(recursive: true);
    }
  }
  
  /// Compile in strong mode. Invalidate a file so type inferrer starts
  
negative.code:   var options = new CompilerOptions()
      ..sdkRoot = sdkRoot
      ..librariesSpecificationUri = Uri.base.resolve("sdk/lib/libraries.json")
      ..onProblem = (LocatedMessage message, Severity severity, String formatted,
          int line, int column) {
        if (severity == Severity.error || severity == Severity.warning) {
          Expect.fail("Unexpected error: $formatted");
        }
      }
      ..strongMode = false;
    return options;
  }
  