Fixing double-completing indexed DB transactions

sdk/lib/indexed_db/dart2js/indexed_db_dart2js.dart

 /**
  * A client-side key-value store with support for indexes.
  *
  * Many browsers support IndexedDB—a web standard for
  * an indexed database.
  * By storing data on the client in an IndexedDB,
  * a web app gets some advantages, such as faster performance and persistence.
  * To find out which browsers support IndexedDB,
  * refer to [Can I Use?](http://caniuse.com/#feat=indexeddb)
  *
@@ skipping 1243 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 74 matching lines @@
 // BSD-style license that can be found in the LICENSE file.
 
 
 @DocsEditable()
 @DomName('IDBAny')
 @deprecated // nonstandard
 abstract class _IDBAny extends Interceptor native "IDBAny" {
   // To suppress missing implicit constructor warnings.
   factory _IDBAny._() { throw new UnsupportedError("Not supported"); }
 }