* first batch of @js bindings

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

* use Object for operator==

* apply style guide for operator==

This set of tests is already running on LUCI.

Bug:
  https://github.com/flutter/flutter/issues/60607

Fixes https://github.com/flutter/flutter/issues/59109