anchor.diff:  class _AnalyzerSourceLoader<L> extends SourceLoader<L> {
     final Map<Uri, Map<Uri, List<CollectedResolution>>> _resolutions;
   
  -  _AnalyzerSourceLoader(
  -      FileSystem fileSystem, TargetImplementation target, this._resolutions)
  +  _AnalyzerSourceLoader(front_end.FileSystem fileSystem,
  +      TargetImplementation target, this._resolutions)
         : super(fileSystem, true, target);
   
     @override
  
positive.code: import 'package:front_end/src/api_prototype/byte_store.dart';
  import 'package:front_end/src/api_prototype/compilation_message.dart';
  import 'package:front_end/src/api_prototype/compiler_options.dart';
  import 'package:front_end/src/api_prototype/file_system.dart';
  import 'package:front_end/src/base/libraries_specification.dart';
  import 'package:front_end/src/base/performance_logger.dart';
  import 'package:front_end/src/base/processed_options.dart';
  
negative.code:   }
  }
  
  /// Adapter of [FileSystemState] to [front_end.FileSystem].
  class _FileSystemAdaptor implements front_end.FileSystem {
    final FileSystemState fsState;
    final pathos.Context pathContext;
  
    _FileSystemAdaptor(this.fsState, this.pathContext);
  
    @override
    front_end.FileSystemEntity entityForUri(Uri uri) {
      if (uri.isScheme('file')) {
        var path = pathContext.fromUri(uri);
        var file = fsState.getFileForPath(path);
        return new _FileSystemEntityAdaptor(uri, file);
      } else {
        throw new front_end.FileSystemException(
            uri, 'Only file:// URIs are supported, but $uri is given.');
      }
    }
  }
  
  /// Adapter of [FileState] to [front_end.FileSystemEntity].
  class _FileSystemEntityAdaptor implements front_end.FileSystemEntity {
    final Uri uri;
    final FileState file;
  
  