investigate adding a command to test a published package

[devoncarew]

This would be something like:

pub global run dart2_fix --check-package foo_bar

​which could:

• check pub.dartlang.org for foo_bar
• see if it had a github repo reference in its homepage: field
• clone that to a local directory
• pub get in that directory
• run dart2_fix there and display any results​