Refactor summary IDL generator.

I need to make changes to 'collectApiSignature' and 'flushInformative'
to support variants, and we have lots of code around already.

[email protected], [email protected]

Change-Id: I9ba3b61a22b9e9e2c547e97d5c5adaa07443b24a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/103552
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov