dart2js cps: Backward redundant null check removal

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 5d56560bed9ac0c9b8b3183ad528a8376c401374..57c9647e15f5b7164bb7aaf2e80bfb3b1c039b82 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
@@ -346,3 +346,17 @@ void destroyAndReplace(Expression node, Expression newNode) {
   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();
+  }
+}