dart2js cps: Rename duplicate_branch.dart to match the new pass name.

pkg/compiler/lib/src/cps_ir/path_based_optimizer.dart

Index: pkg/compiler/lib/src/cps_ir/path_based_optimizer.dart
diff --git a/pkg/compiler/lib/src/cps_ir/duplicate_branch.dart b/pkg/compiler/lib/src/cps_ir/path_based_optimizer.dart
similarity index 99%
rename from pkg/compiler/lib/src/cps_ir/duplicate_branch.dart
rename to pkg/compiler/lib/src/cps_ir/path_based_optimizer.dart
index 26244a462ce1dd062e0a2911f068c2ca97df1efa..6f9fac5aba837c93bca9f7b71f558a9293980728 100644
--- a/pkg/compiler/lib/src/cps_ir/duplicate_branch.dart
+++ b/pkg/compiler/lib/src/cps_ir/path_based_optimizer.dart
@@ -1,7 +1,7 @@
 // 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.
-library dart2js.cps_ir.duplicate_branch; // FIXME: Rename file.
+library dart2js.cps_ir.path_based_optimizer;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart';