[flutter_plugin_tools] Run pub get for custom-test (#5322)

When running a Dart test script for `custom-test`, `pub get` needs to be
run first in order for it to work in a clean environment such as CI.

This will unblock enabling Pigeon's Dart tests in flutter/packages.
4 files changed
tree: 8461d8f85bd7cd2d977c3b520a2aa1552c54f37c
  1. script/