Work around a Firefox computed style bug.

lib/src/runner/browser/static/host.css

 /* 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. */
 
+iframe {
+  /* We would use display: none here, but then Firefox fails to properly compute
+   * styles. See #274 */
+  visibility: hidden;
+}
+
 /* Compiled output from
  * http://codepen.io/mknadler/pen/11b75cb014a3c382f54abf527655af21. */
 
-iframe {
-  display: none;
-}
-
 svg {
   position: absolute;
   margin: auto;
   left: 0;
   right: 0;
   bottom: 0;
   top: 0;
 }
 
 #right-flank {
@@ skipping 236 matching lines @@
     -webkit-transform: translate(110px, 0px) scale(0.4);
             transform: translate(110px, 0px) scale(0.4);
     fill: #00A6E4;
   }
   80% {
     -webkit-transform: translate(0px, 0px) scale(1);
             transform: translate(0px, 0px) scale(1);
     fill: #0074C1;
   }
 }