dart2js cps: Add a pass for eliminating bounds checks.

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

 // Copyright (c) 2015, 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 cps_ir.cps_fragment;
 
 import 'cps_ir_nodes.dart';
 import '../constants/values.dart';
 import '../universe/selector.dart' show Selector;
 import '../types/types.dart' show TypeMask;
@@ skipping 296 matching lines @@
     root.parent = node;
     context.body = child;
     child.parent = context;
     root = context = null;
   }
 
   void insertAbove(InteriorExpression node) {
     insertBelow(node.parent);
   }
 }
+
+/// Removes [node], unlinking all its references and replaces it with [newNode].
+void destroyAndReplace(Expression node, Expression newNode) {
+  InteriorNode parent = node.parent;
+  RemovalVisitor.remove(node);
+  parent.body = newNode;
+  newNode.parent = parent;
+}