Support method cascades

[jacob314]

Library authors have difficulty creating fluent interfaces in Dart because the language lacks built in support for cascading.

Document fully describing the feature request:
https://docs.google.com/document/d/1U0PeHtVQHMQ8usy7xI5Luo01W5LuWR1acN5odgu_Mtw/edit

[gbracha]

Definitely worth looking at. No promises.

---

Added Accepted label.

[DartBot]

This comment was originally written by @mhausner

---

I think this can be closed. We have cascades in the spec now.

[gbracha]

Added Done label.