Prepare plugin repo for binding API improvements.

Since plugins also have to work with the stable branch, we can't actually take advantage of the changes right away, so we muddle the issue in the meantime.

Author: Hixie

packages/url_launcher/url_launcher/lib/url_launcher.dart

@@ -84,10 +84,13 @@ Future<bool> launch(
   bool previousAutomaticSystemUiAdjustment = true;
   if (statusBarBrightness != null &&
       defaultTargetPlatform == TargetPlatform.iOS &&
-      WidgetsBinding.instance != null) {
-    previousAutomaticSystemUiAdjustment =
-        WidgetsBinding.instance!.renderView.automaticSystemUiAdjustment;
-    WidgetsBinding.instance!.renderView.automaticSystemUiAdjustment = false;
+      _ambiguate(WidgetsBinding.instance) != null) {
+    previousAutomaticSystemUiAdjustment = _ambiguate(WidgetsBinding.instance)!
+        .renderView
+        .automaticSystemUiAdjustment;
+    _ambiguate(WidgetsBinding.instance)!
+        .renderView
+        .automaticSystemUiAdjustment = false;
     SystemChrome.setSystemUIOverlayStyle(statusBarBrightness == Brightness.light
         ? SystemUiOverlayStyle.dark
         : SystemUiOverlayStyle.light);
@@ -104,9 +107,11 @@ Future<bool> launch(
     webOnlyWindowName: webOnlyWindowName,
   );
 
-  if (statusBarBrightness != null && WidgetsBinding.instance != null) {
-    WidgetsBinding.instance!.renderView.automaticSystemUiAdjustment =
-        previousAutomaticSystemUiAdjustment;
+  if (statusBarBrightness != null &&
+      _ambiguate(WidgetsBinding.instance) != null) {
+    _ambiguate(WidgetsBinding.instance)!
+        .renderView
+        .automaticSystemUiAdjustment = previousAutomaticSystemUiAdjustment;
   }
 
   return result;
@@ -139,3 +144,10 @@ Future<bool> canLaunch(String urlString) async {
 Future<void> closeWebView() async {
   return await UrlLauncherPlatform.instance.closeWebView();
 }
+
+/// This allows a value of type T or T? to be treated as a value of type T?.
+///
+/// We use this so that APIs that have become non-nullable can still be used
+/// with `!` and `?` on the stable branch.
+// TODO(ianh): Remove this once we roll stable in late 2021.
+T? _ambiguate<T>(T? value) => value;

packages/url_launcher/url_launcher/test/url_launcher_test.dart

@@ -239,7 +239,7 @@ void main() {
         ..setResponse(true);
 
       final TestWidgetsFlutterBinding binding =
-          TestWidgetsFlutterBinding.ensureInitialized()
+          _anonymize(TestWidgetsFlutterBinding.ensureInitialized())
               as TestWidgetsFlutterBinding;
       debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
       binding.renderView.automaticSystemUiAdjustment = true;
@@ -268,7 +268,7 @@ void main() {
         ..setResponse(true);
 
       final TestWidgetsFlutterBinding binding =
-          TestWidgetsFlutterBinding.ensureInitialized()
+          _anonymize(TestWidgetsFlutterBinding.ensureInitialized())
               as TestWidgetsFlutterBinding;
       debugDefaultTargetPlatformOverride = TargetPlatform.android;
       expect(binding.renderView.automaticSystemUiAdjustment, true);
@@ -283,3 +283,11 @@ void main() {
     });
   });
 }
+
+/// This removes the type information from a value so that it can be cast
+/// to another type even if that cast is redundant.
+///
+/// We use this so that APIs whose type have become more descriptive can still
+/// be used on the stable branch where they require a cast.
+// TODO(ianh): Remove this once we roll stable in late 2021.
+Object? _anonymize<T>(T? value) => value;

packages/url_launcher/url_launcher_platform_interface/lib/link.dart

@@ -87,9 +87,10 @@ typedef _SendMessage = Function(String, ByteData?, void Function(ByteData?));
 Future<ByteData> pushRouteNameToFramework(Object? _, String routeName) {
   final Completer<ByteData> completer = Completer<ByteData>();
   SystemNavigator.routeInformationUpdated(location: routeName);
-  final _SendMessage sendMessage =
-      WidgetsBinding.instance?.platformDispatcher.onPlatformMessage ??
-          ui.channelBuffers.push;
+  final _SendMessage sendMessage = _ambiguate(WidgetsBinding.instance)
+          ?.platformDispatcher
+          .onPlatformMessage ??
+      ui.channelBuffers.push;
   sendMessage(
     'flutter/navigation',
     _codec.encodeMethodCall(
@@ -102,3 +103,10 @@ Future<ByteData> pushRouteNameToFramework(Object? _, String routeName) {
   );
   return completer.future;
 }
+
+/// This allows a value of type T or T? to be treated as a value of type T?.
+///
+/// We use this so that APIs that have become non-nullable can still be used
+/// with `!` and `?` on the stable branch.
+// TODO(ianh): Remove this once we roll stable in late 2021.
+T? _ambiguate<T>(T? value) => value;