Implement shrinking reductions in CPS IR

sdk/lib/_internal/compiler/implementation/cps_ir/optimizers.dart

 // Copyright (c) 2014, 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.optimizers;
 
 import 'cps_ir_nodes.dart';
 
-part 'redundant_phi.dart';
+part 'redundant_phi.dart';
+part 'shrinking_reductions.dart';
+
+/// An optimization pass over the CPS IR.
+abstract class Pass {
+  /// Applies optimizations to root, rewriting it in the process.
+  void rewrite(FunctionDefinition root);
+}