Unterminated array error on '{[a,b]}'

Created by: emturner

Reproduction: echo {[a,b]}

Expected behavior: A syntax error or some other kind of error message.

Actual behavior: thread 'main' panicked at 'ion: fatal error with syntax validation: unterminated array expression', src/lib/parser/shell_expand/words/mod.rs:160:9

Build information: [output of rustc -V, git rev-parse HEAD, qemu-i386 -version, uname -a, ion --version, et rustc -V: rustc 1.27.0-nightly (8a37c75a3 2018-05-02) git rev-parse HEAD: 361db0b6