package:build_runner v2.9.0

• Watch mode: handle builder code and config changes without recompiling or
  exiting.
• Remove log output about build_runner internals.
• Print the port that gets picked if you pass 0 for a port number, for example
  with dart run build_runner serve web:0.
• Improve warnings when an option is specified for an unknown builder.
• Rewrite bootstrap code to remove use of dart:mirrors.
• Bug fix: require args 2.5.0.