A mirror on the function associated with this closure.
The function associated with an implicit closure of a function is that function.
The function associated with an instance of a class that has a call
method is that call method.
A Dart implementation might choose to create a class for each closure
expression, in which case function would be the same as
type.declarations[#call]. But the Dart language model does not require
this. A more typical implementation involves a single closure class for
each type signature, where the call method dispatches to a function held
in the closure rather the call method
directly implementing the closure body. So one cannot rely on closures from
distinct closure expressions having distinct classes (type), but one
can rely on them having distinct functions (function).