Clean up imports.

dart/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 '../constants/expressions.dart' show
     ConstantExpression,
     PrimitiveConstantExpression;
 import '../constants/values.dart';
 import '../dart_types.dart' as types;
 import '../dart2jslib.dart' as dart2js;
-import '../elements/elements.dart' show ClassElement;
 import '../tree/tree.dart' show LiteralDartString;
 import '../util/util.dart';
 import 'cps_ir_nodes.dart';
 
 part 'constant_propagation.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);
 }