  We constrain the shape of the Milky Way's halo by dynamical modeling of the
observed phase-space tracks of the Pal 5 and GD-1 tidal streams. We find that
the only information about the potential gleaned from the tracks of these
streams are precise measurements of the shape of the gravitational
potential---the ratio of vertical to radial acceleration---at the location of
the streams, with weaker constraints on the radial and vertical accelerations
separately. The latter will improve significantly with precise proper-motion
measurements from Gaia. We measure that the overall potential flattening is
0.95 +/- 0.04 at the location of GD-1 ([R,z] ~ [12.5,6.7] kpc) and 0.94 +/-
0.05 at the position of Pal 5 ([R,z] ~ [8.4,16.8] kpc). Combined with
constraints on the force field near the Galactic disk, we determine that the
axis ratio of the dark-matter halo's density distribution is 1.05 +/- 0.14
within the inner 20 kpc, with a hint that the halo becomes more flattened near
the edge of this volume. The halo mass within 20 kpc is 1.1 +/- 0.1 x 10^{11}
M_sun. A dark-matter halo this close to spherical is in tension with the
predictions from numerical simulations of the formation of dark-matter halos.
