To declare a dependency on Flutter, from a pub package, add this to your pubspec.yaml:

dependencies:
  flutter:
    sdk: flutter

You must use flutter packages get instead of pub get, because Flutter needs to set the appropriate environment variables to map the sdk: flutter to a location.

To easily and quickly create a new pub package, use the stagehand tool. After you install stagehand, follow these instructions:

$ mkdir fancy_project
$ cd fancy_project
$ stagehand package-simple

After you create your new package, edit the pubspec.yaml and add the sdk:flutter as shown above.

A trick I also use is to just flutter create foo (which creates a full-fledged Flutter app, more than you need) and then rm -rf foo/ios foo/android and edit/remove foo/lib/main.dart, etc.

The stagehand solution from @sethladd is more elegant, but requires having installed stagehand.

flutter create now supports templates. The package template creates exactly that:

flutter create -t package

or to create a plugin that depends on platform-specific code:

flutter create -t plugin