fix #470, string interpolation now respects Dart toString

test/codegen/expect/covariance.js

 dart_library.library('covariance', 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 covariance = Object.create(null);
   const _t = Symbol('_t');
   covariance.Foo$ = dart.generic(T => {
@@ skipping 16 matching lines @@
       })
     });
     return Foo;
   });
   covariance.Foo = covariance.Foo$();
   covariance.Bar = class Bar extends covariance.Foo$(core.int) {
     new() {
       super.new();
     }
     add(x) {
-      core.print(`Bar.add got ${x}`);
+      core.print(dart.str`Bar.add got ${x}`);
       super.add(x);
     }
   };
   dart.setSignature(covariance.Bar, {
     methods: () => ({add: [dart.dynamic, [core.int]]})
   });
   covariance.main = function() {
     let foo = new covariance.Bar();
     foo.add('hi');
   };
   dart.fn(covariance.main);
   // Exports:
   exports.covariance = covariance;
 });