anchor.diff:        .where((v) => v != null)
         .toSet();
   
  -  return new _DependencyNode(project, dependencies);
  +  return new _ProjectDependencies(project, dependencies);
   }
  
positive.code: 
    Map<Project, _DependencyNode> _nodeMap = {};
  
    DependencyGraph._(Set<_DependencyNode> dependencySet) {
      dependencySet.forEach((ds) => _add(ds.project, ds.directDependencies));
    }
  
  
negative.code:     return node;
    }
  }
  class _ProjectDependencies {
    final Project project;
    final Set<Project> directDependencies;
  
    _ProjectDependencies(this.project, this.directDependencies);
  }
  
  class _DependencyNode implements _ProjectDependencies {
    final Project project;
    final Set<_DependencyNode> _dependencies;
    Iterable<Project> get directDependencies =>
  