dart2js cps: Do not inline string interpolation helper.

tests/compiler/dart2js/cps_ir/expected/basic_12.js

 // Expectation for test: 
 // var foo = 42;
 // main() { print(foo); }
 
 function() {
-  var v0 = $.foo, line = v0 === 0 ? 1 / v0 < 0 ? "-0.0" : "" + v0 : "" + v0;
+  var line = H.S($.foo);
   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);
   }
 }