dart2js cps: Generate increment and compound operators.

tests/compiler/dart2js/js_backend_cps_ir_codeUnitAt_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 for constant folding and lowering String.codeUnitAt.
 
 library codeUnitAt_tests;
 
 import 'js_backend_cps_ir.dart';
 
@@ skipping 25 matching lines @@
   var sum = 0;
   for (int i = 0; i < s.length; i++) sum += s.codeUnitAt(i);
   return sum;
 }
 main() {
   print(foo('ABC'));
   print(foo('Hello'));
 }""",r"""
 function(s) {
   var v0 = s.length, sum = 0, i = 0;
-  for (; i < v0; sum = sum + s.charCodeAt(i), i = i + 1)
+  for (; i < v0; sum += s.charCodeAt(i), ++i)
     ;
   return sum;
 }"""),
 ];
 
 
 void main() {
   runTests(tests);
 }