  Hydrogen-poor superluminous supernovae (SLSN-I) are a class of rare and
energetic explosions discovered in untargeted transient surveys in the past
decade. The progenitor stars and the physical mechanism behind their large
radiated energies ($\sim10^{51}$ erg) are both debated, with one class of
models primarily requiring a large rotational energy, while the other requires
very massive progenitors to either convert kinetic energy into radiation via
interaction with circumstellar material (CSM), or engender a pair-instability
explosion. Observing the structure of the CSM around SLSN-I offers a powerful
test of some scenarios, though direct observations are scarce. Here, we present
a series of spectroscopic observations of the SLSN-I iPTF16eh, which reveal
both absorption and time- and frequency-variable emission in the Mg II
resonance doublet. We show that these observations are naturally explained as a
resonance scattering light echo from a circumstellar shell. Modeling the
evolution of the emission, we find a shell radius of 0.1 pc and velocity of
3300 km s$^{-1}$, implying the shell was ejected three decades prior to the
supernova explosion. These properties match theoretical predictions of
pulsational pair-instability shell ejections, and imply the progenitor had a He
core mass of $\sim 50-55~{\rm M}_{\odot}$, corresponding to an initial mass of
$\sim 115~{\rm M}_{\odot}$.
