anchor.diff:  
     StringSource(this._contents, String? fullName, {Uri? uri})
         : fullName = fullName ?? '/test.dart',
  -        uri = uri ?? Uri.file(fullName ?? 'file:///test.dart'),
  +        uri = _computeUri(uri, fullName),
           modificationStamp = DateTime.now().millisecondsSinceEpoch;
   
     @override
  
positive.code: 
    @override
    String toString() => 'StringSource ($fullName)';
  }
  
negative.code: 
  import 'package:analyzer/src/generated/engine.dart' show TimestampedData;
  import 'package:analyzer/src/generated/source.dart';
  import 'package:path/path.dart' as pkg_path;
  
  /// An implementation of [Source] that's based on an in-memory Dart string.
  class StringSource extends Source {
  