  At redshifts beyond z>6, as the mean fraction of neutral hydrogen x_HI in the
intergalactic medium (IGM) increases, the line flux of Lyman alpha (Lya)
emitters can be significantly suppressed, which can result in a decrease in the
observed number of emitters at a given Lya flux. However, cosmological HII
regions surrounding the Lya emitting galaxies alleviate these effects. We use
simple models of the Lya line suppression that incorporate the presence of HII
regions to predict the overall effect of the Lya absorption on the Lya
luminosity function. We find, in agreement with other recent studies, that
unless ionizing sources are unusually strongly clustered, a fully neutral IGM
may be inconsistent with the large abundance of confirmed z=6.5 Lya emitters.
However, the presence of local HII regions prohibits placing a tight constraint
on the mean neutral fraction. We find x_HI < 0.25; the presence of strong winds
and/or the clustering of ionizing sources would further weaken this constraint.
We conclude that the evolution of the Lya LF is consistent with reionization
occurring near this redshift. Finally, we suggest that a measurement of
observed Lya line width as a function of the Lya luminosity, in a future,
larger sample of Lya emitters, may serve as a robust diagnostic of the neutral
fraction in the IGM.
