Automatically append ".js" on spawnUri if we are running in JavaScript.

tests/isolate/spawn_uri_multi_test.dart

 // Copyright (c) 2012, 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.
 
 // Negative test to make sure that we are reaching all assertions.
 // Note: the following comment is used by test.dart to additionally compile the
 // other isolate's code.
 // OtherScripts=spawn_uri_child_isolate.dart
 library spawn_tests;
 import 'dart:isolate';
 import 'package:unittest/unittest.dart';
 
 /* Dummy import so multi-test copies the file.
 import 'spawn_uri_child_isolate.dart';
 */
 
 main() {
   test('isolate fromUri - negative test', () {
     ReceivePort port = new ReceivePort();
     port.receive(expectAsync2((msg, _) {
       String expectedMessage = 're: hi';
       // Should be hi, not hello.
       expectedMessage = 're: hello'; /// 01: runtime error
       expect(msg, equals(expectedMessage));
       port.close();
     }));
 
-    // TODO(eub): make this work for non-JS targets.
-    SendPort s = spawnUri('spawn_uri_child_isolate.js');
+    SendPort s = spawnUri('spawn_uri_child_isolate.dart');
     s.send('hi', port.toSendPort());
   });
 }