Fixing double-completing indexed DB transactions

tools/dom/templates/html/impl/impl_IDBTransaction.darttemplate

 // Copyright (c) 2013, 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.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 
   /**
    * Provides a Future which will be completed once the transaction has
    * completed.
    *
    * The future will error if an error occurrs on the transaction or if the
    * transaction is aborted.
    */
   Future<Database> get completed {
     var completer = new Completer<Database>();
 
     this.onComplete.first.then((_) {
       completer.complete(db);
     });
 
     this.onError.first.then((e) {
       completer.completeError(e);
     });
 
     this.onAbort.first.then((e) {
-      completer.completeError(e);
+      // Avoid completing twice if an error occurs.
+      if (!completer.isCompleted) {
+        completer.completeError(e);
+      }
     });
 
     return completer.future;
   }
 
 $!MEMBERS
 }