dart2js cps: Do not inline string interpolation helper.

tests/compiler/dart2js/cps_ir/expected/basic_14.js

 // Expectation for test: 
 // var foo = 0;
 // main() { print(foo = 42); }
 
 function() {
-  var line = "" + 42;
   $.foo = 42;
   if (typeof dartPrint == "function")
-    dartPrint(line);
+    dartPrint("42");
   else if (typeof console == "object" && typeof console.log != "undefined")
-    console.log(line);
+    console.log("42");
   else if (!(typeof window == "object")) {
     if (!(typeof print == "function"))
-      throw "Unable to print message: " + String(line);
-    print(line);
+      throw "Unable to print message: " + String("42");
+    print("42");
   }
 }