dart2js cps: Add helpers for common IR manipulation.

pkg/compiler/lib/src/cps_ir/remove_refinements.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.optimization.remove_refinements;
 
 import 'optimizers.dart' show Pass;
 import 'shrinking_reductions.dart' show ParentVisitor;
 import 'cps_ir_nodes.dart';
 
 /// Removes all [Refinement] nodes from the IR.
 ///
 /// This simplifies subsequent passes that don't rely on path-sensitive
 /// type information but depend on equality between primitives.
 class RemoveRefinements extends RecursiveVisitor implements Pass {
   String get passName => 'Remove refinement nodes';
 
   void rewrite(FunctionDefinition node) {
     new ParentVisitor().visit(node);
     visit(node);
   }
 
   @override
   Expression traverseLetPrim(LetPrim node) {
+    Expression next = node.body;
     if (node.primitive is Refinement) {
       Refinement refinement = node.primitive;
       refinement.value.definition.substituteFor(refinement);
-      refinement.value.unlink();
-      InteriorNode parent = node.parent;
-      parent.body = node.body;
-      node.body.parent = parent;
+      refinement.destroy();
+      node.remove();
     }
-    return node.body;
+    return next;
   }
 }