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/universe.dart' show Selector;
 import '../types/types.dart' show TypeMask;
 import '../io/source_information.dart';
 import '../elements/elements.dart';
+import 'shrinking_reductions.dart' show ParentVisitor;
 
 /// Builds a CPS fragment that can be plugged into another CPS term.
 ///
 /// A CPS fragment contains a CPS term, possibly with a "hole" in it denoting
 /// where to insert new IR nodes. We say a fragment is "open" if it has such
 /// a hole. Otherwise, the fragment is "closed" and cannot be extended further.
 ///
 /// This class is designed for building non-trivial CPS terms in a readable and
 /// non-error prone manner. It is not designed to manipulate existing IR nodes,
 /// nor is it intended to shield the user from every complexity in the IR.
@@ skipping 268 matching lines @@
     letPrim(new SetMutable(variable, value));
   }
 
   /// Declare a new mutable variable.
   void letMutable(MutableVariable variable, Primitive initialValue) {
     LetMutable let = new LetMutable(variable, initialValue);
     put(let);
     context = let;
   }
 }
+
+/// Removes [node], unlinking all its references and replaces it with [newNode],
+/// setting all parent pointers in [newNode].
+///
+/// Be careful not to pass in a [newNode] containing a existing IR subtree as
+/// this will generally cause expensive redundant reprocessing.
+void replaceExpression(Expression node, Expression newNode) {
+  InteriorNode parent = node.parent;
+  RemovalVisitor.remove(node);
+  parent.body = newNode;
+  newNode.parent = parent;
+  ParentVisitor.setParentPointers(newNode);
+}
+