Format record type annotations.

[kallentu]

As title says, formats record type annotations.

• Used a DelimitedListBuilder for these since they work similarly to parameter lists, but not quite the same.
• Added a few comment tests, just to make sure they work.

The comments show how the splitting has changed for these, specifically in the case of:

>>> Split positional types.
(TypeName,TypeName,TypeName,TypeName) x;
<<<
(
  TypeName,
  TypeName,
  TypeName,
  TypeName,
) x;

which no longer splits like:

(
  TypeName,
  TypeName,
  TypeName,
  TypeName,
)
x;

[munificent]

This is fantastic. The tests are excellent. I made a few suggests on the code, but it's all minor stuff. This is a nice Christmas present to come back to. :)

[kallentu]

Sending to @sigmundch for readability comments. Please review as a readability CL and add your feedback at go/dart-team-readability-feedback

[sigmundch]

LGTM!