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

lib/src/runner/browser/server.dart

Index: lib/src/runner/browser/server.dart
diff --git a/lib/src/runner/browser/server.dart b/lib/src/runner/browser/server.dart
index 9c58885e1f404e794c461a85a730934a383975a4..2a8d08e8e4f9ad4881296edb4d06eed0b149400b 100644
--- a/lib/src/runner/browser/server.dart
+++ b/lib/src/runner/browser/server.dart
@@ -353,7 +353,7 @@ void main() {
     // value and [browser.onError].
     _browserManagers[platform] = completer.future.catchError((_) {});
     var path = _webSocketHandler.create(webSocketHandler((webSocket) {
-      completer.complete(new BrowserManager(webSocket));
+      completer.complete(new BrowserManager(platform, webSocket));
     }));
 
     var webSocketUrl = url.replace(scheme: 'ws').resolve(path);