Specify asserts in initialization lists

[munificent]

We have an informal specification:

https://github.com/dart-lang/sdk/blob/master/docs/language/informal/assert-in-initializer-list.md

But that only works on casual Fridays. For the remainder of the week, we need a formal suit and tie specification.

[lrhn]

This specification landed in 609d26a + a72f46d.

[munificent]

Awesome, thanks!