The element that is used to write the result.
In invalid code, for recovery, we might use other elements, for example a
myGetter = 0 even though the getter
cannot be used to write a value. We do this to help the user to navigate
to the getter, and maybe add the corresponding setter.
This element is
null if the AST structure has not been resolved, or
if the target could not be resolved.
Element get writeElement;