[ddc] Make it possible to move all dart.applyMixin() calls to a library link method

[nshahan]

There is a need to move all class hierarchy connections into a single method in each library so they can be linked after all class definitions have been evaluated. Initially, this meant removing the use of the extends keyword in compiled output and start stitching together prototype chains manually later in the evaluation of a library.

All calls of dart.applyMixin() need to be moved as well so that when new classes are created (this happens in the body of the method) at the time of a mixin application they have the correct superclass set before copying members from one class to another.

The representation of generic type parameters is also intertwined with mixins in some class hierarchies. This is preventing simply moving all calls of dart.applyMixin() to the link method. It might be easier to change the generic class representation first. #31003