Fixing double-completing indexed DB transactions

sdk/lib/indexed_db/dartium/indexed_db_dartium.dart

 library dart.dom.indexed_db;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:html_common';
 import 'dart:nativewrappers';
 // 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.
 
@@ skipping 931 matching lines @@
 
     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;
   }
 
   // To suppress missing implicit constructor warnings.
   factory Transaction._() { throw new UnsupportedError("Not supported"); }
 
   @DomName('IDBTransaction.abortEvent')
   @DocsEditable()
@@ skipping 88 matching lines @@
 
 
 @DocsEditable()
 @DomName('IDBAny')
 @deprecated // nonstandard
 abstract class _IDBAny extends NativeFieldWrapperClass1 {
   // To suppress missing implicit constructor warnings.
   factory _IDBAny._() { throw new UnsupportedError("Not supported"); }
 
 }