Add web startup analyzer to material 3 demo (#2144)

This adds a tool to measure web app startup for the Material 3 demo.

Demo:
- [Example app](https://flutter-web-perf-experiments.web.app/)
- [Material
3](https://flutter-web-perf-experiments--material3-vswzldcy.web.app/)
(open console)

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>
Co-authored-by: Kevin Moore <kevmoo@google.com>

web/_tool/verify_packages.dart

@@ -11,6 +11,7 @@ import 'common.dart';
 void main() async {
   final packageDirs = listPackageDirs(Directory.current)
       .map((path) => p.relative(path, from: Directory.current.path))
+      .where((path) => !p.dirname(path).startsWith('_'))
       .toList();
 
   print('Package dirs:\n${packageDirs.map((path) => '  $path').join('\n')}');