drscheme:unit:add-to-program-editor-mixin in DrScheme Tools Functions

• (drscheme:unit:add-to-program-editor-mixin mixin) -> void?
     mixin : ((subclass?/c text%) . -> . (subclass?/c text%))

  This function can only be called in phase 1.

  Adds mixin to the result of drscheme:unit:get-program-editor-mixin.