We need the ability to efficiently transform a mixin application class
into a regular class, and the separation was obstructing it. It also
simplifies the IR a bit, although it no longer enforces the invariant
that mixin applications cannot contain fields and procedures.
The binary format is unchanged, so the separation still shows up in
there. This ensures the CL can land without any changes to the SDK.
BUG=
R=vegorov@google.com
Review URL: https://chromereviews.googleplex.com/488407013 .
Asger Feldthaus