  We report the detection and mass measurement of a binary lens
OGLE-2015-BLG-1285La,b, with the more massive component having
$M_1>1.35\,M_\odot$ (80% probability). A main-sequence star in this mass range
is ruled out by limits on blue light, meaning that a primary in this mass range
must be a neutron star or black hole. The system has a projected separation
$r_\perp= 6.1\pm 0.4\,{\rm AU}$ and lies in the Galactic bulge. These
measurements are based on the "microlens parallax" effect, i.e., comparing the
microlensing light curve as seen from $Spitzer$, which lay at $1.25\,{\rm AU}$
projected from Earth, to the light curves from four ground-based surveys, three
in the optical and one in the near infrared. Future adaptive optics imaging of
the companion by 30m class telescopes will yield a much more accurate
measurement of the primary mass. This discovery both opens the path and defines
the challenges to detecting and characterizing black holes and neutron stars in
wide binaries, with either dark or luminous companions. In particular, we
discuss lessons that can be applied to future $Spitzer$ and $Kepler$ K2
microlensing parallax observations.
