Fixing double-completing indexed DB transactions

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

Index: tools/dom/templates/html/impl/impl_IDBTransaction.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_IDBTransaction.darttemplate b/tools/dom/templates/html/impl/impl_IDBTransaction.darttemplate
index 180b2b0555e9976f73af7fc64411c9bce45e9350..39e9d30cc0eb39b2848f93b7df87e2afa685d493 100644
--- a/tools/dom/templates/html/impl/impl_IDBTransaction.darttemplate
+++ b/tools/dom/templates/html/impl/impl_IDBTransaction.darttemplate
@@ -16,16 +16,22 @@ $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   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;