dart2js cps: Do not inline string interpolation helper.

tests/compiler/dart2js/cps_ir/expected/interceptors_1.js

Index: tests/compiler/dart2js/cps_ir/expected/interceptors_1.js
diff --git a/tests/compiler/dart2js/cps_ir/expected/interceptors_1.js b/tests/compiler/dart2js/cps_ir/expected/interceptors_1.js
index ed58ff02800990e78b02ee63049211e28575d50e..a8df9d12d709101a452827b4e0f016d0bfc62193 100644
--- a/tests/compiler/dart2js/cps_ir/expected/interceptors_1.js
+++ b/tests/compiler/dart2js/cps_ir/expected/interceptors_1.js
@@ -7,14 +7,13 @@
 // }
 
 function() {
-  var line = "" + 4;
   if (typeof dartPrint == "function")
-    dartPrint(line);
+    dartPrint("4");
   else if (typeof console == "object" && typeof console.log != "undefined")
-    console.log(line);
+    console.log("4");
   else if (!(typeof window == "object")) {
     if (!(typeof print == "function"))
-      throw "Unable to print message: " + String(line);
-    print(line);
+      throw "Unable to print message: " + String("4");
+    print("4");
   }
 }