package:build_runner v2.7.0

• Performance: builders can choose to run only when "triggered". A builder runs
  only if triggered if the option run_only_if_triggered is true. Triggers
  are configured in new a top-level section of build.yaml called triggers.
  See the build_config docs
  for more information.
• Remove interactive prompts for whether to delete files.
• Ignore -d flag: always delete files as if -d was passed.
• Remove --use-polling-watcher flag.
• Remove build_runner serve mode $graph and $perf handlers.
• Documentation revamp.
• Bug fix: delete transitive generated outputs as well as direct generated
  outputs. So, a generated file now gets deleted if its input was a generated
  file that is no longer output.
• Bug fix: fix crash creating a merged output dir with a file called lib.