fix #470, string interpolation now respects Dart toString

test/codegen/expect/async_helper.js

 dart_library.library('async_helper', 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 async_helper = Object.create(null);
   async_helper._initialized = false;
   async_helper._Action0 = dart.typedef('_Action0', () => dart.functionType(dart.void, []));
   async_helper._onAsyncEnd = null;
   async_helper._asyncLevel = 0;
   async_helper._buildException = function(msg) {
-    return core.Exception.new(`Fatal: ${msg}. This is most likely a bug in your test.`);
+    return core.Exception.new(dart.str`Fatal: ${msg}. This is most likely a bug in your test.`);
   };
   dart.fn(async_helper._buildException, core.Exception, [core.String]);
   async_helper.asyncTestInitialize = function(callback) {
     async_helper._asyncLevel = 0;
     async_helper._initialized = false;
     async_helper._onAsyncEnd = callback;
   };
   dart.fn(async_helper.asyncTestInitialize, dart.void, [async_helper._Action0]);
   dart.copyProperties(async_helper, {
     get asyncTestStarted() {
@@ skipping 36 matching lines @@
   };
   dart.fn(async_helper.asyncSuccess, dart.void, [dart.dynamic]);
   async_helper.asyncTest = function(f) {
     async_helper.asyncStart();
     dart.dsend(f(), 'then', async_helper.asyncSuccess);
   };
   dart.fn(async_helper.asyncTest, dart.void, [dart.functionType(dart.dynamic, [])]);
   // Exports:
   exports.async_helper = async_helper;
 });