fix #577 - correctly toString ES Symbols

test/browser/runtime_tests.js

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 var assert = chai.assert;
 var dart_sdk = dart_library.import('dart_sdk');
 var core = dart_sdk.core;
 var collection = dart_sdk.collection;
 var dart = dart_sdk.dart;
 var dartx = dart.dartx;
@@ skipping 1145 matching lines @@
 });
 
 suite('primitives', function() {
   'use strict';
 
   test('fixed length list', () => {
     let list = new core.List(10);
     list[0] = 42;
     assert.throws(() => list.add(42));
   });
+
+  test('toString on ES Symbol', () => {
+    let sym = Symbol('_foobar');
+    assert.equal(dart.toString(sym), 'Symbol(_foobar)');
+  });
 });