Don't double-complete in host.dart.

lib/src/runner/browser/static/host.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.
 
 library test.runner.browser.host;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:html';
 
@@ skipping 108 matching lines @@
     // unit tests, let alone be able to find the test server while it's
     // running, but it's good practice to check the origin anyway.
     if (message.origin != window.location.origin) return;
 
     // TODO(nweiz): Stop manually checking href here once issue 22554 is
     // fixed.
     if (message.data["href"] != iframe.src) return;
 
     message.stopPropagation();
     inputController.add(message.data["data"]);
-    readyCompleter.complete();
+    if (!readyCompleter.isCompleted) readyCompleter.complete();
   });
 
   outputController.stream.listen((message) {
     readyCompleter.future.then((_) =>
         iframe.contentWindow.postMessage(message, window.location.origin));
   });
 
   return new StreamChannel(inputController.stream, outputController.sink);
 }