dart2js cps: Update null checks when updating refinements.

pkg/compiler/lib/src/js_backend/codegen/task.dart

Index: pkg/compiler/lib/src/js_backend/codegen/task.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/task.dart b/pkg/compiler/lib/src/js_backend/codegen/task.dart
index e89c4aee4f063038e50bcb970e9a47ba21df64c1..f6a8f101a5ad10d10d16599b402324c71a96d60b 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/task.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/task.dart
@@ -222,6 +222,7 @@ class CpsFunctionCompiler implements FunctionCompiler {
       applyCpsPass(new RedundantJoinEliminator(), cpsFunction);
       applyCpsPass(new ShrinkingReducer(), cpsFunction);
       applyCpsPass(new RedundantRefinementEliminator(typeSystem), cpsFunction);
+      applyCpsPass(new UpdateRefinements(typeSystem), cpsFunction);
       applyCpsPass(new TypePropagator(this, recomputeAll: true), cpsFunction);
       applyCpsPass(new ShrinkingReducer(), cpsFunction);
       applyCpsPass(new EagerlyLoadStatics(), cpsFunction);