dart2js cps: Do not inline string interpolation helper.

tests/compiler/dart2js/cps_ir/expected/basic_9.js

 // Expectation for test: 
 // class C {}
 // main() {
 //   print(new C());
 // }
 
 function() {
-  var res = "Instance of '" + H.Primitives_objectTypeName(V.C$()) + "'";
+  var line = "Instance of '" + H.Primitives_objectTypeName(V.C$()) + "'";
   if (typeof dartPrint == "function")
-    dartPrint(res);
+    dartPrint(line);
   else if (typeof console == "object" && typeof console.log != "undefined")
-    console.log(res);
+    console.log(line);
   else if (!(typeof window == "object")) {
     if (!(typeof print == "function"))
-      throw "Unable to print message: " + String(res);
-    print(res);
+      throw "Unable to print message: " + String(line);
+    print(line);
   }
 }