[test.dart] Fix configuration override with one configuration

Bug: https://github.com/dart-lang/sdk/issues/56660
Change-Id: I9831b51cb2493789f8d9dc662ebc06cfd2410fce
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/384744
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Auto-Submit: Alexander Thomas <athom@google.com>
diff --git a/pkg/test_runner/lib/test_runner.dart b/pkg/test_runner/lib/test_runner.dart
index 028a7ab..07cde72 100644
--- a/pkg/test_runner/lib/test_runner.dart
+++ b/pkg/test_runner/lib/test_runner.dart
@@ -440,7 +440,7 @@
     var inexactBuilds = <String, String>{};
     var previousFileName = "previous.json";
     var flakyFileName = "flaky.json";
-    var downloadNumber = 0;
+    var downloadNumber = needsMerge ? 0 : 1;
 
     // Download the previous results and flakiness info from cloud storage.
     for (var builder in builders) {