dart2js cps: Compile some loops as 'for' loops.

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 68 matching lines @@
   var a;
   for (var i=0; i<10; i++) {
     a = () => i;
   }
   print(a());
 }
 """,
 r"""
 function() {
   var a = null, i = 0;
-  while (i < 10) {
-    a = new V.main_closure(i);
-    i = i + 1;
-  }
+  for (; i < 10; a = new V.main_closure(i), i = i + 1)
+    ;
   P.print(a.call$0());
 }"""),
 
   const TestEntry.forMethod('function(A#b)', """
 class A {
   a() => 1;
   b() => () => a();
 }
 main() {
   print(new A().b()());
@@ skipping 81 matching lines @@
 """,
 r"""
 function(x) {
   P.print(V.Foo$().getter$1(123));
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }