Pull JS assignments into var initializer.

tests/compiler/dart2js/js_backend_cps_ir_supercall_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 interceptors.
 
 library supercall_test;
 
 import 'js_backend_cps_ir.dart';
@@ skipping 46 matching lines @@
   var field;
 }
 class Sub extends Base {
   m(x) => x + super.field;
 }
 main() {
   print(new Sub().m(10));
 }""",
 r"""
 function(x) {
-  var v0;
-  v0 = this.field;
+  var v0 = this.field;
   return J.getInterceptor$ns(x).$add(x, v0);
 }"""),
 
 
 ];
 
 void main() {
   runTests(tests);
 }