dart2js cps: Maintain parent pointers instead of recomputing them.

pkg/compiler/lib/src/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.cps_ir.optimizers;
 
 import 'cps_ir_nodes.dart';
 import '../constants/values.dart';
 
 export 'type_propagation.dart' show TypePropagator;
 export 'scalar_replacement.dart' show ScalarReplacer;
 export 'redundant_phi.dart' show RedundantPhiEliminator;
 export 'redundant_join.dart' show RedundantJoinEliminator;
-export 'shrinking_reductions.dart' show ShrinkingReducer, ParentVisitor;
+export 'shrinking_reductions.dart' show ShrinkingReducer;
 export 'mutable_ssa.dart' show MutableVariableEliminator;
 export 'insert_refinements.dart' show InsertRefinements;
 export 'remove_refinements.dart' show RemoveRefinements;
 export 'share_interceptors.dart' show ShareInterceptors;
+export 'parent_visitor.dart' show ParentVisitor;
 
 /// An optimization pass over the CPS IR.
 abstract class Pass {
   /// Applies optimizations to root, rewriting it in the process.
   void rewrite(FunctionDefinition root);
 
   String get passName;
 }
 
 // Shared code between optimizations
 
 /// Returns true if [value] is false, null, 0, -0, NaN, or the empty string.
 bool isFalsyConstant(ConstantValue value) {
   return value.isFalse ||
       value.isNull  ||
       value.isZero ||
       value.isMinusZero ||
       value.isNaN ||
       value is StringConstantValue && value.primitiveValue.isEmpty;
 }
 
 /// Returns true if [value] satisfies a branching condition with the
 /// given strictness.
 ///
 /// For non-strict, this is the opposite of [isFalsyConstant].
 bool isTruthyConstant(ConstantValue value, {bool strict: false}) {
   return strict ? value.isTrue : !isFalsyConstant(value);
 }