Fix initialization of type parameter bounds on mixin applications.
Fix initialization of type parameter bounds on mixin applications.

There was a circularity issue with type parameter bounds being copied
from other classes, while those classes we not yet fully initialized.

BUG=
R=kmillikin@google.com

Review URL: https://chromereviews.googleplex.com/499437013 .

dart-lang/kernel,dart-archive/kernel