Fixes for Dart 2 failures in tests (#18371)

packages/flutter_tools/lib/src/android/android_device.dart

@@ -510,7 +510,7 @@ class AndroidDevice extends Device {
   bool get supportsScreenshot => true;
 
   @override
-  Future<Null> takeScreenshot(File outputFile) async {
+  Future<void> takeScreenshot(File outputFile) async {
     const String remotePath = '/data/local/tmp/flutter_screenshot.png';
     await runCheckedAsync(adbCommandForDevice(<String>['shell', 'screencap', '-p', remotePath]));
     await runCheckedAsync(adbCommandForDevice(<String>['pull', remotePath, outputFile.path]));

packages/flutter_tools/lib/src/device.dart

@@ -277,7 +277,7 @@ abstract class Device {
 
   bool get supportsScreenshot => false;
 
-  Future<Null> takeScreenshot(File outputFile) => new Future<Null>.error('unimplemented');
+  Future<void> takeScreenshot(File outputFile) => new Future<Null>.error('unimplemented');
 
   /// Find the apps that are currently running on this device.
   Future<List<DiscoveredApp>> discoverApps() =>

packages/flutter_tools/lib/src/ios/mac.dart

@@ -98,7 +98,7 @@ class IMobileDevice {
   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]);
   }
 }

packages/flutter_tools/lib/src/ios/simulators.dart

@@ -137,7 +137,7 @@ class SimControl {
     return runCheckedAsync(args);
   }
 
-  Future<Null> takeScreenshot(String deviceId, String outputPath) {
+  Future<void> takeScreenshot(String deviceId, String outputPath) {
     return runCheckedAsync(<String>[_xcrunPath, 'simctl', 'io', deviceId, 'screenshot', outputPath]);
   }
 }
@@ -458,7 +458,7 @@ class IOSSimulator extends Device {
   bool get supportsScreenshot => _xcodeVersionSupportsScreenshot;
 
   @override
-  Future<Null> takeScreenshot(File outputFile) {
+  Future<void> takeScreenshot(File outputFile) {
     return SimControl.instance.takeScreenshot(id, outputFile.path);
   }
 }