Use better names for captured variables in closures.

tests/compiler/dart2js/js_backend_cps_ir_closures_test.dart

Index: tests/compiler/dart2js/js_backend_cps_ir_closures_test.dart
diff --git a/tests/compiler/dart2js/js_backend_cps_ir_closures_test.dart b/tests/compiler/dart2js/js_backend_cps_ir_closures_test.dart
index 820be3ea3e5e50700ec0dd3c6aa75e8de650b990..be1bd40f01b37e3daede7ef65f823fd3782a99d3 100644
--- a/tests/compiler/dart2js/js_backend_cps_ir_closures_test.dart
+++ b/tests/compiler/dart2js/js_backend_cps_ir_closures_test.dart
@@ -36,8 +36,8 @@ main(x) {
 r"""
 function(x) {
   var _box_0 = {}, a = new V.main_a(_box_0);
-  _box_0._captured_x_0 = x;
-  _box_0._captured_x_0 = J.getInterceptor$ns(x = _box_0._captured_x_0).$add(x, "1");
+  _box_0.x = x;
+  _box_0.x = J.getInterceptor$ns(x = _box_0.x).$add(x, "1");
   P.print(a.call$0());
   return a;
 }"""),
@@ -96,8 +96,8 @@ main() {
 r"""
 function() {
   var _box_0 = {}, a = new V.main_closure(_box_0);
-  _box_0._captured_x_0 = 122;
-  _box_0._captured_x_0 = _box_0._captured_x_0 + 1;
+  _box_0.x = 122;
+  _box_0.x = _box_0.x + 1;
   P.print(a.call$0());
   return a;
 }"""),
@@ -133,8 +133,8 @@ main() {
 r"""
 function() {
   var _box_0 = {}, a = new V.main_closure(_box_0);
-  _box_0._captured_x_0 = 122;
-  _box_0._captured_x_0 = _box_0._captured_x_0 + 1;
+  _box_0.x = 122;
+  _box_0.x = _box_0.x + 1;
   P.print(a.call$0().call$0());
   return a;
 }"""),