Add support to dart2js for option --enable-asserts.

tools/testing/dart/test_configurations.dart

Index: tools/testing/dart/test_configurations.dart
diff --git a/tools/testing/dart/test_configurations.dart b/tools/testing/dart/test_configurations.dart
index 9a75efdeced36477d176e7a79489db228b581973..f49ccf0f259766740247c23e0323ecd5e915fa60 100644
--- a/tools/testing/dart/test_configurations.dart
+++ b/tools/testing/dart/test_configurations.dart
@@ -104,6 +104,7 @@ Future testConfigurations(List<Configuration> configurations) async {
       if (configuration.isChecked) settings.add('checked');
       if (configuration.isStrong) settings.add('strong');
       if (configuration.useFastStartup) settings.add('fast-startup');
+      if (configuration.useEnableAsserts) settings.add('enable-asserts');
       outputWords.add(settings.join('_'));
     }
     print(outputWords.join(' '));