dart2js cps: Do not inline string interpolation helper.

tests/compiler/dart2js/cps_ir/expected/codeUnitAt_1.js

 // Expectation for test: 
 // // Constant folding
 // main() {
 //   print('A'.codeUnitAt(0));
 // }
 
 function() {
-  var line = "" + 65;
   if (typeof dartPrint == "function")
-    dartPrint(line);
+    dartPrint("65");
   else if (typeof console == "object" && typeof console.log != "undefined")
-    console.log(line);
+    console.log("65");
   else if (!(typeof window == "object")) {
     if (!(typeof print == "function"))
-      throw "Unable to print message: " + String(line);
-    print(line);
+      throw "Unable to print message: " + String("65");
+    print("65");
   }
 }