Pull JS assignments into var initializer.

tests/compiler/dart2js/js_backend_cps_ir_control_flow_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.
 // VMOptions=-DUSE_CPS_IR=true
 
 // Tests of control flow statements.
 
 library control_flow_tests;
 
 import 'js_backend_cps_ir.dart';
@@ skipping 39 matching lines @@
 foo(a) => a;
 
 main() {
   for (int i = 0; foo(true); i = foo(i)) {
     print(1);
     if (foo(false)) break;
   }
   print(2);
 }""", """
 function() {
-  var i;
-  i = 0;
+  var i = 0;
   L1:
     while (true) {
       if (P.identical(V.foo(true), true)) {
         P.print(1);
         if (!P.identical(V.foo(false), true)) {
           i = V.foo(i);
           continue L1;
         }
       }
       P.print(2);
@@ skipping 64 matching lines @@
 function() {
   V.foo();
   P.print("good");
   return null;
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }