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 cecad3191c8bbc8f295d11f3cc67b0f8b5d9a3c6..7fc2ae38373fb2bd8cac048dd568cec129d9ae34 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
@@ -314,3 +314,11 @@ class CpsFragment {
     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;
+}