dart2js cps: Do not inline string interpolation helper.

tests/compiler/dart2js/cps_ir/expected/supercall_3.js

 // Expectation for test: 
 // class Base {
 //   var field = 123;
 // }
 // class Sub extends Base {
 //   m(x) => x + super.field;
 // }
 // main() {
 //   print(new Sub().m(10));
 // }
 
 function() {
-  var line = "" + (10 + V.Sub$().field);
+  var line = H.S(10 + V.Sub$().field);
   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);
   }
 }