[flutter_tool] Screenshot command must require device only for _kDeviceType (#44227)

There are cases where we have access to the observatory without having a device connection accessible.

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

@@ -63,18 +63,27 @@ class ScreenshotCommand extends FlutterCommand {
 
   Device device;
 
-  @override
-  Future<FlutterCommandResult> verifyThenRunCommand(String commandPath) async {
-    device = await findTargetDevice();
+  static void validateOptions(String screenshotType, Device device, String observatoryUri) {
+    switch (screenshotType) {
+      case _kDeviceType:
         if (device == null) {
-      throwToolExit('Must have a connected device');
+          throwToolExit('Must have a connected device for screenshot type $screenshotType');
         }
-    if (argResults[_kType] == _kDeviceType && !device.supportsScreenshot) {
+        if (!device.supportsScreenshot) {
           throwToolExit('Screenshot not supported for ${device.name}.');
         }
-    if (argResults[_kType] != _kDeviceType && argResults[_kObservatoryUri] == null) {
-      throwToolExit('Observatory URI must be specified for screenshot type ${argResults[_kType]}');
+        break;
+      default:
+        if (observatoryUri == null) {
+          throwToolExit('Observatory URI must be specified for screenshot type $screenshotType');
+        }
     }
+  }
+
+  @override
+  Future<FlutterCommandResult> verifyThenRunCommand(String commandPath) async {
+    device = await findTargetDevice();
+    validateOptions(argResults[_kType], device, argResults[_kObservatoryUri]);
     return super.verifyThenRunCommand(commandPath);
   }
 

packages/flutter_tools/test/general.shard/screenshot_command_test.dart

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_tools/src/commands/screenshot.dart';
+
+import '../src/common.dart';
+import '../src/context.dart';
+
+void main() {
+  group('Validate screenshot options', () {
+    testUsingContext('rasterizer and skia screenshots do not require a device', () async {
+      ScreenshotCommand.validateOptions('rasterizer', null, 'dummy_observatory_uri');
+      ScreenshotCommand.validateOptions('skia', null, 'dummy_observatory_uri');
+    });
+
+    testUsingContext('rasterizer and skia screenshots require observatory uri', () async {
+      expect(() => ScreenshotCommand.validateOptions('rasterizer', null, null), throwsToolExit());
+      expect(() => ScreenshotCommand.validateOptions('skia', null, null), throwsToolExit());
+    });
+
+    testUsingContext('device screenshots require device', () async {
+      expect(() => ScreenshotCommand.validateOptions('device', null, null), throwsToolExit());
+    });
+  });
+}