Consider Uri SDK variables

[stuartmorgan-g]

This is minor, but since the example header entry point uses: packageRoot.resolve('lib/add.g.dart'), once I found the SDK variables that replaced the magic strings from the YAML config I tried to do macSdkPath.resolve(...), and discovered that they were Strings, not Uris. Since the main (only?) use is to pass them to an API that requires Uris, it seems like it would be more user-friendly to provide them as Uris.

[liamappelbe]

Trying to avoid breaking changes now, so I'll add new variables, eg macSdkUri.