* fix gclient runhooks for windows

* remove usage of dartdev command so that gclient runhooks can re-generate package_config.json

Generate package config during gclient runhooks and update BUILD.gn to reference it

The dart SDK needs to ship several packages which have opted into null safety for the tech preview. This requires the use of a package_config.json for compilation as this file contains the language versions. Use a dart script to generate the package_config file from the .packages file, and include that in the DEPS hooks section.

Fixes flutter/flutter#60628