Create a Dart SDK auto-roller

[cbracken]

Create an auto-roller that periodically checks for a new Dart SDK version, sends a PR to upgrade the linux and macos versions, and lands it if all tests pass.

[cbracken]

Added tools/roll_dart in 486b9c0 and 84627e3. This automates the process of generating the diff required to roll the Dart SDK version in repositories.bzl and README.md.

Remaining work:

• Clean up roll_dart. It was hacked together in an afternoon to reduce the manual work involved in rolling but isn't excactly nice code.
• The roll command should run tests via tools/run_tests.sh and if successsful, generate a new branch and commit with a version comment.
• Add a tag command or some other mechanism that generates a signed SDK version tag and pushes.

It would be nice to have this on a cron job that:

• looks up the latest Dart SDK handled in this repo
• looks up the latest Dart SDK on stable track
• if there is a new Dart SDK on stable track, checks if there is any pending PR for that version and if not, creates one.