dart2js cps: Add helpers for common IR manipulation.

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

Index: pkg/compiler/lib/src/cps_ir/remove_refinements.dart
diff --git a/pkg/compiler/lib/src/cps_ir/remove_refinements.dart b/pkg/compiler/lib/src/cps_ir/remove_refinements.dart
index f5088ae214008351cbb595d737c6578bd1ba4478..4d7aae3d847ccd9a76dd70788a1867e713e706dd 100644
--- a/pkg/compiler/lib/src/cps_ir/remove_refinements.dart
+++ b/pkg/compiler/lib/src/cps_ir/remove_refinements.dart
@@ -22,14 +22,13 @@ class RemoveRefinements extends RecursiveVisitor implements Pass {
 
   @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;
   }
 }