Use better names for captured variables in closures.

tests/compiler/dart2js/js_backend_cps_ir_closures_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Tests of closures.
 
 library closures_test;
 
 import 'js_backend_cps_ir.dart';
 
@@ skipping 18 matching lines @@
     return x;
   }
   x = x + '1';
   print(a());
   return a;
 }
 """,
 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;
 }"""),
 
   const TestEntry("""
 main(x) {
   a() {
     return x;
   }
   print(a());
@@ skipping 38 matching lines @@
   var x = 122;
   var a = () => x;
   x = x + 1;
   print(a());
   return a;
 }
 """,
 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;
 }"""),
 
   const TestEntry("""
 main() {
   var x = 122;
   var a = () {
     var y = x;
     return () => y;
@@ skipping 15 matching lines @@
     return () => y;
   };
   x = x + 1;
   print(a()());
   return a;
 }
 """,
 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;
 }"""),
 
   const TestEntry("""
 main() {
   var a;
   for (var i=0; i<10; i++) {
     a = () => i;
   }
@@ skipping 99 matching lines @@
 """,
 r"""
 function(x) {
   P.print(V.Foo$().getter$1(123));
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }