Add tests for do expressions

[Daniel Tschinder]

We currently have not a single test for doExpressions. We should add some tests to babylon.
Can take the examples from the official gist.

Tests should go into test/fixtures/experimental/do-expressions/

https://gist.github.com/dherman/1c97dfb25179fa34a41b5fff040f9879

[Sangboak Lee]

Let me tackle this issue 🖐

[Justin Ridgewell]

Closed by #688.