Suppress get_drt/dartium download if an executable is provided via flag

tools/testing/dart/test_options.dart

Index: tools/testing/dart/test_options.dart
diff --git a/tools/testing/dart/test_options.dart b/tools/testing/dart/test_options.dart
index d730d728143f75fa7bd82ef03bda6fe713919675..1eedf00a2f347e5090a2fbadc19576573bfc8d9a 100644
--- a/tools/testing/dart/test_options.dart
+++ b/tools/testing/dart/test_options.dart
@@ -518,7 +518,7 @@ is 'dart file.dart' and you specify special command
       return _expandHelper('runtime', configuration);
     } else {
       // All runtimes eventually go through this path, after expansion.
-      var updater = runtimeUpdater(runtimes);
+      var updater = runtimeUpdater(configuration);
       if (updater !== null) {
         updater.update();
       }