dart2js cps: Eliminate unary string concatenation.

tests/compiler/dart2js/js_backend_cps_ir_operators_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 operators.
 
 library operators_tests;
 
 import 'js_backend_cps_ir.dart';
 
@@ skipping 112 matching lines @@
 //}
 //"""),
 
 const TestEntry("""
 main() {
   var list = [1, 2, 3];
   list[1] = 6;
   print(list);
 }""", r"""
 function() {
-  var list = [1, 2, 3], res, v0;
+  var list = [1, 2, 3], v0;
   list[1] = 6;
-  if (!(typeof (res = C.JSArray_methods.toString$0(list)) === "string"))
+  if (!(typeof (v0 = P.IterableBase_iterableToFullString(list, "[", "]")) === "string"))
     throw H.wrapException(H.argumentErrorValue(list));
-  v0 = res;
   if (typeof dartPrint == "function")
     dartPrint(v0);
   else if (typeof console == "object" && typeof console.log != "undefined")
     console.log(v0);
   else if (!(typeof window == "object")) {
     if (!(typeof print == "function"))
       throw "Unable to print message: " + String(v0);
     print(v0);
   }
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }