Add an "onPlatform" parameter to test() and group().

lib/src/runner/loader.dart

Index: lib/src/runner/loader.dart
diff --git a/lib/src/runner/loader.dart b/lib/src/runner/loader.dart
index 4b1f3d7447c1fe29874be04299603522abdc4156..a6db18d7904bd404930fe26633a5eaae728f9b4a 100644
--- a/lib/src/runner/loader.dart
+++ b/lib/src/runner/loader.dart
@@ -148,9 +148,7 @@ class Loader {
         assert(platform.isBrowser);
         return _loadBrowserFile(path, platform, metadata);
       }).then((suite) {
-        if (suite == null) return;
-
-        controller.add(suite.filter(platform, os: currentOS));
+        if (suite != null) controller.add(suite);
       }).catchError(controller.addError);
     }).then((_) => controller.close());