  The pairing symmetry is examined in highly electron-doped
Ba(Fe$_{1-x}$Co$_x$As)$_2$ and A$_y$Fe$_2$Se$_2$ (with A=K, Cs) compounds, with
similar crystallographic and electronic band structures. Starting from a
phenomenological two-orbital model, we consider nearest-neighbor and
next-nearest-neighbor intraorbital pairing interactions on the Fe square
lattice. In this model, we find a unified description of the evolution from
$s_\pm$-wave pairing ($2.0 < n \lesssim 2.4$) to $d$-wave pairing ($2.4
\lesssim n \lesssim 2.5$) as a function of electron filling. In the crossover
region a novel time-reversal symmetry breaking state with $s_\pm+id$ pairing
symmetry emerges. This minimal model offers an overall picture of the evolution
of superconductivity with electron doping for both $s_\pm$-wave
[Ba(Fe$_{1-x}$Co$_x$As)$_2$] and $d$-wave [A$_y$Fe$_2$Se$_2$] pairing, as long
as the dopants only play the role of a charge reservoir. However, the situation
is more complicated for Ba(Fe$_{1-x}$Co$_x$As)$_2$. A real-space study further
shows that when the impurity scattering effects of Co dopants are taken into
account, the superconductivity is completely suppressed for $n > 2.4$. This
preempts any observation of $d$-wave pairing in this compound, in contrast to
A$_y$Fe$_2$Se$_2$.
