Add missing copyrights

pkg/front_end/lib/src/dependency_walker.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
 
 /**
  * Instances of [Node] represent nodes in a dependency graph.  The
  * type parameter, [NodeType], is the derived type (this affords some
  * extra type safety by making it difficult to accidentally construct
  * bridges between unrelated dependency graphs).
  */
 abstract class Node<NodeType> {
   /**
    * Index used by Tarjan's strongly connected components algorithm.
@@ skipping 145 matching lines @@
           }
           evaluateScc(scc);
         }
       }
     }
 
     // Kick off the algorithm starting with the starting point.
     strongConnect(startingPoint);
   }
 }