anchor.diff:    _FileSystemViewEntry(this.uri, this.file);
   
     @override
  -  Future<bool> exists() async => file?.exists ?? false;
  -
  -  @override
  -  Future<DateTime> lastModified() async {
  -    throw new StateError(
  -        'FileSystemViewEntry modification stamp should not be queried');
  -  }
  +  Future<bool> exists() async => _shouldNotBeQueried();
  +
  +  @override
  +  Future<DateTime> lastModified() async => _shouldNotBeQueried();
   
     @override
     Future<List<int>> readAsBytes() async {
  -    _throwIfDoesNotExist();
  -    return file.contentBytes;
  -  }
  -
  -  @override
  -  Future<String> readAsString() async {
  -    _throwIfDoesNotExist();
  -    return file.content;
  -  }
  -
  -  void _throwIfDoesNotExist() {
       if (file == null) {
         throw new FileSystemException(uri, 'File $uri does not exist.');
       }
  -  }
  -}
  +    return file.content;
  +  }
  +
  +  @override
  +  Future<String> readAsString() async => _shouldNotBeQueried();
  +
  +  /// _FileSystemViewEntry is used by the incremental kernel generator to
  +  /// provide Fasta with a consistent, race condition free view of the files
  +  /// constituting the project.  It should only need to be used for reading
  +  /// file contents.
  +  dynamic _shouldNotBeQueried() {
  +    throw new StateError('The method should not be invoked.');
  +  }
  +}
  
positive.code:     FileState file = await _fsState.getFile(resolvedUri);
      files.add(file);
    }
  }
  
  /// Information about known file system state.
  
negative.code: // BSD-style license that can be found in the LICENSE file.
  
  import 'dart:async';
  import 'dart:typed_data';
  
  import 'package:front_end/file_system.dart';
  