Use better names for captured variables in closures.

tests/compiler/dart2js/js_backend_cps_ir_constructor_test.dart

 // Copyright (c) 2015, 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 interceptors.
 
 library constructor_test;
 
 import 'js_backend_cps_ir.dart';
 
@@ skipping 78 matching lines @@
   Sub(x, this.y) : super(x) {
     print(x);
   }
 }
 main() {
   print(new Sub(1, 2).x);
 }""",
 r"""
 function(x, y) {
   var _box_0 = {}, v0 = new V.Sub(y, new V.Base_closure(_box_0));
-  _box_0._captured_x1_0 = x;
+  _box_0.x1 = x;
   v0.Base0$0();
   v0.Base$1(_box_0);
   v0.Sub$2(x, y);
   return v0;
 }"""),
 
   const TestEntry.forMethod('generative_constructor(Sub#)', """
 foo(x) {
   print(x);
 }
@@ skipping 149 matching lines @@
   new A(5).typevar;
 }""", r"""
 function() {
   V.B$(5, P.$int).get$typevar();
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }