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
method is that
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 (
MethodMirror get function;