Fix BoundsCheck copying.

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 782 matching lines @@
   static const int UPPER_BOUND = 1 << 0;
   static const int LOWER_BOUND = 1 << 1;
   static const int EMPTINESS = 1 << 2; // See [hasEmptinessCheck].
   static const int BOTH_BOUNDS = UPPER_BOUND | LOWER_BOUND;
   static const int NONE = 0;
 
   BoundsCheck(Primitive object, Primitive index, Primitive length,
       [this.checks = BOTH_BOUNDS, this.sourceInformation])
       : this.object = new Reference<Primitive>(object),
         this.index = new Reference<Primitive>(index),
-        this.length = new Reference<Primitive>(length);
+        this.length = length == null ? null : new Reference<Primitive>(length);
 
   BoundsCheck.noCheck(Primitive object, [this.sourceInformation])
       : this.object = new Reference<Primitive>(object),
         this.checks = NONE;
 
   accept(Visitor visitor) => visitor.visitBoundsCheck(this);
 
   void setParentPointers() {
     object.parent = this;
     if (index != null) {
@@ skipping 1858 matching lines @@
   Definition visitRefinement(Refinement node) {
     return new Refinement(getCopy(node.value), node.refineType);
   }
 
   Definition visitBoundsCheck(BoundsCheck node) {
     if (node.hasNoChecks) {
       return new BoundsCheck.noCheck(getCopy(node.object),
           node.sourceInformation);
     } else {
       return new BoundsCheck(getCopy(node.object), getCopy(node.index),
-          getCopy(node.length),
+          node.length == null ? null : getCopy(node.length),
           node.checks,
           node.sourceInformation);
     }
   }
 
   Definition visitNullCheck(NullCheck node) {
     return new NullCheck(getCopy(node.value), node.sourceInformation,
         condition: node.condition == null ? null : getCopy(node.condition),
         selector: node.selector,
         useSelector: node.useSelector);
@@ skipping 137 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());
   }
 }