[Extension type] Write the spec

[itsjustkevin]

[nuritzi]

Per @eernstg in go/dart-feature-cheatsheet: the spec is still a WIP; the two current proposals are: views and extension structs (We'll do one, or the other, or a combination)

[eernstg]

This feature is now known as 'inline classes'. The spec has been written, accepted by the language team, and can be found here: https://github.com/dart-lang/language/blob/master/accepted/future-releases/inline-classes/feature-specification.md.