  We present the distance priors that we have derived from the 2015 Planck
data, and use these in combination with the latest observational data from Type
Ia Supernovae (SNe Ia) and galaxy clustering, to explore the systematic
uncertainties in dark energy constraints. We use the Joint Lightcurve Analysis
(JLA) set of 740 SNe Ia, galaxy clustering measurements of H(z)s and D_A(z)/s
(where s is the sound horizon at the drag epoch) from the Sloan Digital Sky
Survey (SDSS) at z=0.35 and z=0.57 (BOSS DR12). We find that the combined dark
energy constraints are insensitive to the assumptions made in the galaxy
clustering measurements (whether they are for BAO only or marginalized over
RSD), which indicates that as the analysis of galaxy clustering data becomes
more accurate and robust, the systematic uncertainties are reduced. On the
other hand, we find that flux-averaging SNe Ia at z>= 0.5 significantly
tightens the dark energy constraints, and excludes a flat universe with a
cosmological constant at 68% confidence level, assuming a dark energy equation
of state linear in the cosmic scale factor. Flux-averaging has the most
significant effect when we allow dark energy density function X(z) to be a free
functions given by the cubic spline of its value at z=0, 1/3, 2/3, 1; the
measured X(z) deviates from a cosmological constant at more than 95% confidence
level for 0.4 < z < 0.7. Since flux-averaging reduces the bias in the SN
distance measurements, this may be an indication that we have arrived in the
era when the SN distance measurements are limited by systematic uncertainties.
