fix #470, string interpolation now respects Dart toString

test/codegen/expect/map_keys.js

 dart_library.library('map_keys', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const math = dart_sdk.math;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const map_keys = Object.create(null);
   map_keys.main = function() {
     core.print(dart.map({'1': 2, '3': 4, '5': 6}));
     core.print(dart.map([1, 2, 3, 4, 5, 6]));
-    core.print(dart.map({'1': 2, [`${dart.notNull(math.Random.new().nextInt(2)) + 2}`]: 4, '5': 6}));
+    core.print(dart.map({'1': 2, [dart.str`${dart.notNull(math.Random.new().nextInt(2)) + 2}`]: 4, '5': 6}));
     let x = '3';
     core.print(dart.map(['1', 2, x, 4, '5', 6]));
     core.print(dart.map(['1', 2, null, 4, '5', 6]));
   };
   dart.fn(map_keys.main);
   // Exports:
   exports.map_keys = map_keys;
 });
 
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