[CI] Add a roll-to-Dart-SDK job

It would be nice if we can add a roll-to-Dart-SDK job that:

1. Makes a CL for the Dart SDK changing the DEPS file to
a. point the repository to GitHub instead of Dart mirror, and
b. sets the commit hash to the PRs commit hash.
2. Fires of the 5 package bots that run the tests ([see current test results](https://dart-current-results.web.app/#/filter=native_toolchain_c/,native_assets_builder/,native_assets_cli/&showAll)).

I have no idea how hard or easy this would be. cc @athomas @whesse Do we have anything like this already? How hard would this be?

This would prevent us breaking rolls into the Dart SDK. And this might be useful for other DEPS as well that roll into the Dart SDK as an early warning sign. cc @devoncarew