Work around a Firefox computed style bug.

test/runner/browser/firefox_html_test.dart

+// 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.
+
+@TestOn("firefox")
+
+import 'dart:html';
+
+import 'package:test/test.dart';
+
+void main() {
+  // Regression test for #274. Firefox doesn't compute styles within hidden
+  // iframes (https://bugzilla.mozilla.org/show_bug.cgi?id=548397), so we have
+  // to do some special stuff to make sure tests that care about that work.
+  test("getComputedStyle() works", () {
+    expect(document.body.getComputedStyle(), isNotNull);
+  });
+}