[testing] Create list from test list iterable

This fixes a bug in test.py's new --tests option.

Change-Id: I0410972116045c612429565081ad84735d2d2bf1
Reviewed-on: https://dart-review.googlesource.com/c/92784
Auto-Submit: Alexander Thomas <athom@google.com>
Commit-Queue: William Hesse <whesse@google.com>
Reviewed-by: William Hesse <whesse@google.com>
diff --git a/tools/testing/dart/options.dart b/tools/testing/dart/options.dart
index 0f0a789..8593b59 100644
--- a/tools/testing/dart/options.dart
+++ b/tools/testing/dart/options.dart
@@ -513,7 +513,7 @@
       if (configuration.containsKey('test_list_contents')) {
         _fail('--tests and --test-list cannot be used together');
       }
-      configuration['test_list_contents'] = LineSplitter.split(tests);
+      configuration['test_list_contents'] = LineSplitter.split(tests).toList();
     }
 
     return _createConfigurations(configuration);