Don't rely on script.async in deferred loading.

dart/sdk/lib/_internal/lib/async_patch.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.
 
 // Patch file for the dart:async library.
 
 import 'dart:_isolate_helper' show IsolateNatives, TimerImpl;
 import 'dart:_foreign_helper' show JS, DART_CLOSURE_TO_JS;
 
 patch Timer _createTimer(Duration duration, void callback()) {
@@ skipping 53 matching lines @@
         int index = uri.lastIndexOf('/');
         uri = '${uri.substring(0, index + 1)}part.js';
       }
       // Create a new function to avoid getting access to current function
       // context.
       JS('void', '(new Function(#))()', 'load("$uri")');
       return true;
     });
   }
 
-  Completer completer = new Completer<bool>();
-  _loadedLibraries[libraryName] = completer.future;
-  try {
+  return _loadedLibraries[libraryName] = new Future<bool>(() {
+    Completer completer = new Completer<bool>();
     if (uri == null) {
       uri = IsolateNatives.thisScript;
       int index = uri.lastIndexOf('/');
       uri = '${uri.substring(0, index + 1)}part.js';
     }
 
     // Inject a script tag.
     var script = JS('', 'document.createElement("script")');
     JS('', '#.type = "text/javascript"', script);
-    JS('', '#.async = "async"', script);
     JS('', '#.src = #', script, uri);
     var onLoad = JS('', '#.bind(null, #)',
                     DART_CLOSURE_TO_JS(_onDeferredLibraryLoad), completer);
     JS('', '#.addEventListener("load", #, false)', script, onLoad);
     JS('', 'document.body.appendChild(#)', script);
-  } catch (e, trace) {
-    completer.completeError(e, trace);
-  }
-  return completer.future;
+
+    return completer.future;
+  });
 }
 
 /// Used to implement deferred loading. Used as callback on "load"
 /// event above in [load].
 _onDeferredLibraryLoad(Completer<bool> completer, event) {
   completer.complete(true);
 }
 
 bool get _hasDocument => JS('String', 'typeof document') == 'object';