Fix return types of commands that override verifyThenRunCommand (#19483)

Fixes https://github.com/flutter/flutter/issues/19465

packages/flutter_tools/lib/src/commands/logs.dart

@@ -29,7 +29,7 @@ class LogsCommand extends FlutterCommand {
   Device device;
 
   @override
-  Future<Null> verifyThenRunCommand() async {
+  Future<FlutterCommandResult> verifyThenRunCommand() async {
     device = await findTargetDevice();
     if (device == null)
       throwToolExit(null);

packages/flutter_tools/lib/src/commands/screenshot.dart

@@ -64,7 +64,7 @@ class ScreenshotCommand extends FlutterCommand {
   Device device;
 
   @override
-  Future<Null> verifyThenRunCommand() async {
+  Future<FlutterCommandResult> verifyThenRunCommand() async {
     device = await findTargetDevice();
     if (device == null)
       throwToolExit('Must have a connected device');