anchor.diff:  
     _TopLevelNameDeclarationsTask(this.driver, this.name);
   
  -  /**
  -   * Perform a single piece of work, and either complete the [completer] and
  -   * return `true` to indicate that the task is done, return `false` to indicate
  -   * that the task should continue to be run.
  -   */
  +  /// Perform a single piece of work, and either complete the [completer] and
  +  /// return `true` to indicate that the task is done, return `false` to indicate
  +  /// that the task should continue to be run.
     bool perform() {
       // Prepare files to check.
       if (filesToCheck.isEmpty) {
  
positive.code:     return unitResult.element.library;
    }
  
    /**
     * Return a [ParsedLibraryResult] for the library with the given [path].
     *
     * Throw [ArgumentError] if the given [path] is not the defining compilation
     * unit for a library (that is, is a part of a library).
     *
     * The [path] must be absolute and normalized.
     */
    ParsedLibraryResult getParsedLibrary(String path) {
      FileState file = _fsState.getFileForPath(path);
  
  
negative.code:   }
  }
  
  /// Information about an exception and its context.
  class _ExceptionState {
    final exception;
    final StackTrace stackTrace;
  
    /// The key under which the context of the exception was stored, or `null`
    /// if unknown, the maximum number of context to store was reached, etc.
    final String contextKey;
  
    _ExceptionState(this.exception, this.stackTrace, this.contextKey);
  