fix #470, string interpolation now respects Dart toString

test/codegen/expect/script.js

 dart_library.library('script', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const script = Object.create(null);
   script.main = function(args) {
     let name = args[dartx.join](' ');
     if (name == '') name = 'world';
-    core.print(`hello ${name}`);
+    core.print(dart.str`hello ${name}`);
   };
   dart.fn(script.main, dart.void, [core.List$(core.String)]);
   // Exports:
   exports.script = script;
 });