  We model the sample of fast radio bursts (FRB), including the newly
discovered CHIME repeaters, using the synchrotron blast wave model of Metzger,
Margalit & Sironi (2019). This model postulates that FRBs are precursor
radiation from ultra-relativistic magnetized shocks generated as flare ejecta
from a central engine collide with an effectively stationary external medium.
Downward drifting of the burst frequency structure naturally arises from
deceleration of the blast-wave. The data are consistent with FRBs being
produced by flares of energy $E_{\rm flare} \sim
10^{43}-10^{46}(f_{\xi}/10^{-3})^{-4/5}$ erg, where $f_{\xi}$ is the maser
efficiency, and minimum bulk Lorentz factors $\Gamma \approx 10^2-10^3$, which
generate the observed FRBs at shock radii $r_{\rm sh} \sim 10^{12}-10^{13}$ cm.
We infer upstream densities $n_{\rm ext}(r_{\rm sh}) \sim 10^{2}-10^{4}$
cm$^{-3}$ and radial profiles $n_{\rm ext} \propto r^{-k}$ showing a range of
slopes $k \approx [-2,1]$ (which are seen to evolve between bursts), broadly
consistent with the upstream medium being the inner edge of an ion-loaded shell
released by a recent energetic flare. The burst timescales, energetics, rates,
and external medium properties are consistent with repeating FRBs arising from
young, hyper-active flaring magnetars, but the methodology presented is
generally applicable to any central engine which injects energy impulsively
into a dense magnetized medium. Uncertainties and variations of the model are
discussed, including the effects of the strong electric field of the FRB wave
(strength parameter $a \gg 1$) on the upstream medium. One-dimensional
particle-in-cell simulations of magnetized shocks into a pair plasma are
presented which demonstrate that high maser efficiency can be preserved, even
in the limit $a \gg 1$ in which the FRB wave accelerates the upstream electrons
to ultra-relativistic speeds.
