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

tests/isolate/spawn_uri_negative_test.dart

Index: tests/isolate/spawn_uri_negative_test.dart
diff --git a/tests/isolate/spawn_uri_negative_test.dart b/tests/isolate/spawn_uri_negative_test.dart
index 6e5be3beceb45f139183f4a5f215582801c78219..b744eeb40b0ece2a797af5bf2174eb504456ce64 100644
--- a/tests/isolate/spawn_uri_negative_test.dart
+++ b/tests/isolate/spawn_uri_negative_test.dart
@@ -18,8 +18,7 @@ main() {
       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());
   });
 }