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

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

 library dart2js.cps_ir.backward_null_check_remover;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart';
 import '../common/names.dart';
 import '../universe/selector.dart';
 import 'type_mask_system.dart';
 import 'cps_fragment.dart';
 
 /// Removes null checks that are follwed by another instruction that will
@@ skipping 59 matching lines @@
 
   /// It has been determined that the null check in [prim] made redundant by
   /// [newNullCheck].  Eliminate [prim] if it is not needed any more.
   void tryEliminateRedundantNullCheck(Primitive prim, Primitive newNullCheck) {
     if (prim is NullCheck) {
       Primitive value = prim.value.definition;
       LetPrim let = prim.parent;
       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();
         let.remove();
       }
     }
   }
 
   /// True if [prim] can be moved above a null check.  This is safe if [prim]
   /// cannot throw or have side effects and does not carry any path-sensitive
@@ skipping 29 matching lines @@
   void visitLetHandler(LetHandler node) {
     nullCheckedValue = null;
   }
 
   visitInvokeContinuation(InvokeContinuation node) {
     if (!node.isRecursive) {
       nullCheckedValue = nullCheckedValueAt[node.continuation.definition];
     }
   }
 }