In the FIR generator, the `AbstractElement` class was used to
represent either an element type without type arguments applied
(using the `Element` subclass), or an element type with applied type
arguments (using the `ElementWithArguments` subclass).
Instead, it is more logical to use the `Element` class to always
represent a non-parameterized element type, and for a parameterized
element type use the `ElementRef` class, just like we do
in the IR generator.
Sergej Jaskiewicz
04ac4b71a3