dart2js cps: Debugging utility and fix idempotency in shrinking reducer.

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

Index: pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
diff --git a/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart b/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
index 422aaa3102216e2b1b64722f8b965714e1a700b7..b55a433ffa2da6d15b7a9fd0eb3b67f4ec1c2265 100644
--- a/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
+++ b/pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart
@@ -77,7 +77,7 @@ class BackwardNullCheckRemover extends BlockVisitor implements Pass {
       prim..replaceUsesWith(value)..destroy();
       let.remove();
     } else if (prim is GetLength || prim is GetField || prim is GetIndex) {
-      if (prim.hasNoEffectiveUses) {
+      if (prim.hasNoRefinedUses) {
         destroyRefinementsOfDeadPrimitive(prim);
         LetPrim let = prim.parent;
         prim..destroy();