dart2js cps: Backward redundant null check removal

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 328 matching lines @@
   }
 }
 
 /// 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;
 }
+
+/// Removes all [Refinement] uses of a given primitive that has no effective
+/// uses.
+void destroyRefinementsOfDeadPrimitive(Primitive prim) {
+  while (prim.firstRef != null) {
+    Refinement refine = prim.firstRef.parent;
+    destroyRefinementsOfDeadPrimitive(refine);
+    LetPrim letPrim = refine.parent;
+    InteriorNode parent = letPrim.parent;
+    parent.body = letPrim.body;
+    letPrim.body.parent = parent;
+    prim.firstRef.unlink();
+  }
+}