dart2js cps: Add a pass for eliminating bounds checks.

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

Index: pkg/compiler/lib/src/cps_ir/cps_fragment.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_fragment.dart b/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
index a9190ddf77aea155692d8abd5577be719ce2a22e..1932a722a759a117aa053576c98bb75553fdd67c 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
@@ -10,6 +10,7 @@ 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.
 ///
@@ -298,3 +299,17 @@ class CpsFragment {
     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);
+}
+