Fixes for Dart 2 failures in tests (#18371)

diff --git a/packages/flutter_tools/lib/src/ios/mac.dart b/packages/flutter_tools/lib/src/ios/mac.dart
index 9539577..5c8db0d 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -98,7 +98,7 @@
   Future<Process> startLogger() => runCommand(<String>['idevicesyslog']);
 
   /// Captures a screenshot to the specified outputFile.
-  Future<Null> takeScreenshot(File outputFile) {
+  Future<void> takeScreenshot(File outputFile) {
     return runCheckedAsync(<String>['idevicescreenshot', outputFile.path]);
   }
 }