Add gerrit mirrors for dartdoc dev_dependencies

[srawlins]

I think the new ones required for dartdoc development are:

• https://github.com/dart-lang/build, containing the build, build_runner, build_test, and build_version packages. These require
  • build_config, build_daemon, build_resolvers, build_runner_core (all found in the same repo),
  • https://github.com/dart-lang/code_builder (also a direct dependency; see below),
  • frontend_server_client in the already mirrored https://github.com/dart-lang/webdev repo
  • graphs, in the already mirrored https://github.com/dart-lang/tools repo
  • https://github.com/dart-lang/io,
  • https://github.com/dart-lang/pubspec_parse,
  • shelf_web_socket, in the already mirrored https://github.com/dart-lang/shelf repo
  • https://github.com/dart-lang/stream_transform
  • https://github.com/dart-lang/timing
• https://github.com/dart-lang/code_builder containing the code_builder package.
  • This package requires built_collection and built_value, which require no new packages.

[srawlins]

Work on this can be held. I think I've been able to remove dependencies on package:build* in dart-lang/dartdoc#3448, and I think I can remove the dependency on package:code_builder.