anchor.diff:      return null;
     }
   
  -  /**
  -   * We detected that one of the required `dart` libraries is missing.
  -   * Return the empty analysis result with the error.
  -   */
  +  /// We detected that one of the required `dart` libraries is missing.
  +  /// Return the empty analysis result with the error.
     AnalysisResult _newMissingDartLibraryResult(
         FileState file, String missingUri) {
       // TODO(scheglov) Find a better way to report this.
  
positive.code:     return completer.future;
    }
  
    /**
     * Return a [Future] that completes with the signature for the
     * [UnitElementResult] for the file with the given [path], or with `null` if
     * the file cannot be analyzed.
     *
     * The signature is based the APIs of the files of the library (including
     * the file itself) of the requested file and the transitive closure of files
     * imported and exported by the library.
     */
    Future<String> getUnitElementSignature(String path) {
      _throwIfNotAbsolutePath(path);
      if (!_fsState.hasUri(path)) {
  
negative.code:     }
    }
  
    /// Notify that there is a change to the [driver], it it might need to
    /// perform some work.
    void notify(AnalysisDriverGeneric driver) {
      _hasWork.notify();
      _statusSupport.preTransitionToAnalyzing();
    }
  
    /// Remove the given [driver] from the scheduler, so that it will not be
    /// asked to perform any new work.
    void remove(AnalysisDriverGeneric driver) {
      if (driver is AnalysisDriver) {
        driverWatcher?.removedDriver(driver);
  