dart2js cps: Do not inline string interpolation helper.

tests/compiler/dart2js/cps_ir/expected/closures_7.js

 // Expectation for test: 
 // main() {
 //   var x = 122;
 //   var a = () {
 //     var y = x;
 //     return () => y;
 //   };
 //   x = x + 1;
 //   print(a()());
 // }
 
 function() {
-  var _captured_x_0 = 122 + 1, line = _captured_x_0 === 0 ? 1 / _captured_x_0 < 0 ? "-0.0" : "" + _captured_x_0 : "" + _captured_x_0;
+  var line = H.S(122 + 1);
   if (typeof dartPrint == "function")
     dartPrint(line);
   else if (typeof console == "object" && typeof console.log != "undefined")
     console.log(line);
   else if (!(typeof window == "object")) {
     if (!(typeof print == "function"))
       throw "Unable to print message: " + String(line);
     print(line);
   }
 }