Uncontroversial and backwards compatible 2.0 SDK fixes (#13723)

Small code changes as part of moving the framework SDK forward to a 2.0 dev version.
diff --git a/examples/platform_channel/lib/main.dart b/examples/platform_channel/lib/main.dart
index f258120..ffae082 100644
--- a/examples/platform_channel/lib/main.dart
+++ b/examples/platform_channel/lib/main.dart
@@ -40,7 +40,7 @@
     eventChannel.receiveBroadcastStream().listen(_onEvent, onError: _onError);
   }
 
-  void _onEvent(String event) {
+  void _onEvent(Object event) {
     setState(() {
       _chargingStatus =
           "Battery status: ${event == 'charging' ? '' : 'dis'}charging.";