Panic on invalid export usage

Ion version

ion 1.0.0-alpha (x86_64-unknown-linux-gnu)
rev 38957cfac84daee8d39a919b76089ca3804c2f46

This script panics:

# coredump.ion
export foo:hmap[] = [ ]

Output

thread 'main' panicked at 'internal error: entered unreachable code', src/lib/shell/assignments.rs:99:30
note: Run with `RUST_BACKTRACE=1` for a backtrace.
[1]    3686 abort (core dumped)  ion coredump.ion