dart2js cps: Clean up in type propagation.

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';
 
-export 'type_propagation.dart' show TypePropagator, TypeSystem,
-                                    UnitTypeSystem, TypeMaskSystem;
+export 'type_propagation.dart' show TypePropagator;
 export 'redundant_phi.dart' show RedundantPhiEliminator;
 export 'shrinking_reductions.dart' show ShrinkingReducer, 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;
 }