Fixing double-completing indexed DB transactions

sdk/lib/indexed_db/dartium/indexed_db_dartium.dart

Index: sdk/lib/indexed_db/dartium/indexed_db_dartium.dart
diff --git a/sdk/lib/indexed_db/dartium/indexed_db_dartium.dart b/sdk/lib/indexed_db/dartium/indexed_db_dartium.dart
index 34dcf6bb979a12ca536deda4020d0bd2c26bc7e1..32ddc6cab49290b6dcefd6187c0943e7b987c8de 100644
--- a/sdk/lib/indexed_db/dartium/indexed_db_dartium.dart
+++ b/sdk/lib/indexed_db/dartium/indexed_db_dartium.dart
@@ -940,16 +940,22 @@ class Transaction extends EventTarget {
   Future<Database> get completed {
     var completer = new Completer<Database>();
 
+    var errored = false;
+
     this.onComplete.first.then((_) {
       completer.complete(db);
     });
 
     this.onError.first.then((e) {
+      errored = true;
       completer.completeError(e);
     });
 
     this.onAbort.first.then((e) {
-      completer.completeError(e);
+      // Avoid completing twice if an error occurs.
+      if (!errored) {
+        completer.completeError(e);
+      }
     });
 
     return completer.future;