Precache iOS artifacts (#49010)

dev/devicelab/bin/tasks/platform_view_ios__start_up.dart

@@ -19,7 +19,20 @@ Future<void> main() async {
     );
     await inDirectory(platformViewDirectory, () async {
       await flutter('pub', options: <String>['get']);
+      // Pre-cache the iOS artifacts; this may be the first test run on this machine.
+      await flutter(
+        'precache',
+        options: <String>[
+          '--no-android',
+          '--no-fuchsia',
+          '--no-linux',
+          '--no-macos',
+          '--no-web',
+          '--no-windows',
+        ],
+      );
     });
+
     final Directory iosDirectory = dir(
       '$platformViewDirectoryPath/ios',
     );