  Measurements of the dark energy equation-of-state parameter, $w$, have been
limited by uncertainty in the selection effects and photometric calibration of
$z<0.1$ Type Ia supernovae (SNe Ia). The Foundation Supernova Survey is
designed to lower these uncertainties by creating a new sample of $z<0.1$ SNe
Ia observed on the Pan-STARRS system. Here, we combine the Foundation sample
with SNe from the Pan-STARRS Medium Deep Survey and measure cosmological
parameters with 1,338 SNe from a single telescope and a single, well-calibrated
photometric system. For the first time, both the low-$z$ and high-$z$ data are
predominantly discovered by surveys that do not target pre-selected galaxies,
reducing selection bias uncertainties. The $z>0.1$ data include 875 SNe without
spectroscopic classifications and we show that we can robustly marginalize over
CC SN contamination. We measure Foundation Hubble residuals to be fainter than
the pre-existing low-$z$ Hubble residuals by $0.046 \pm 0.027$ mag (stat+sys).
By combining the SN Ia data with cosmic microwave background constraints, we
find $w=-0.938 \pm 0.053$, consistent with $\Lambda$CDM. With 463
spectroscopically classified SNe Ia alone, we measure $w=-0.933\pm0.061$. Using
the more homogeneous and better-characterized Foundation sample gives a 55%
reduction in the systematic uncertainty attributed to SN Ia sample selection
biases. Although use of just a single photometric system at low and high
redshift increases the impact of photometric calibration uncertainties in this
analysis, previous low-$z$ samples may have correlated calibration
uncertainties that were neglected in past studies. The full Foundation sample
will observe up to 800 SNe to anchor the LSST and WFIRST Hubble diagrams.
