dart2js cps: Do not remove unused refinements.

pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart

 // Copyright (c) 2013, 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.ir_nodes;
 
 import 'dart:collection';
 import 'cps_fragment.dart' show CpsFragment;
 import '../constants/values.dart' as values;
 import '../dart_types.dart' show DartType, InterfaceType, TypeVariableType;
 import '../elements/elements.dart';
@@ skipping 714 matching lines @@
 /// but need to reason about value references being identical (i.e. referring
 /// to the same primitive).
 class Refinement extends Primitive {
   Reference<Primitive> value;
   final TypeMask refineType;
 
   Refinement(Primitive value, this.refineType)
     : value = new Reference<Primitive>(value);
 
   bool get hasValue => true;
-  bool get isSafeForElimination => true;
+  bool get isSafeForElimination => false;
   bool get isSafeForReordering => false;
 
   accept(Visitor visitor) => visitor.visitRefinement(this);
 
   Primitive get effectiveDefinition => value.definition.effectiveDefinition;
 
   void setParentPointers() {
     value.parent = this;
   }
 }
@@ skipping 2048 matching lines @@
     plug(new Branch.loose(_definitions.getCopy(node.condition),
         _copies[node.trueContinuation.definition],
         _copies[node.falseContinuation.definition])
       ..isStrictCheck = node.isStrictCheck);
   }
 
   visitUnreachable(Unreachable node) {
     plug(new Unreachable());
   }
 }