Add missing copyrights

pkg/front_end/test/src/dependency_walker_test.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.
+
 import 'package:front_end/src/dependency_walker.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(DependencyWalkerTest);
   });
 }
 
@@ skipping 195 matching lines @@
     for (var v in scc) {
       v.isEvaluated = true;
     }
     var sccNames = scc.map((node) => node._name).toSet();
     // Make sure there were no duplicates
     expect(sccNames.length, scc.length);
     _evaluations.add(sccNames);
     _sccFlags.add(true);
   }
 }