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

pkg/compiler/lib/src/cps_ir/path_based_optimizer.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.
-library dart2js.cps_ir.duplicate_branch; // FIXME: Rename file.
+library dart2js.cps_ir.path_based_optimizer;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart';
 import 'cps_fragment.dart';
 import '../js_backend/js_backend.dart';
 import '../constants/values.dart';
 import '../elements/elements.dart';
 import '../universe/selector.dart';
 import '../types/types.dart';
 import 'type_mask_system.dart';
@@ skipping 160 matching lines @@
             node.selector)) {
         Constant dummy = new Constant(new IntConstantValue(0))
             ..type = typeSystem.intType;
         new LetPrim(dummy).insertAbove(node.parent);
         node.arguments[0].changeTo(dummy);
         node.callingConvention = CallingConvention.DummyIntercepted;
       }
     }
   }
 }