Calling an alias that uses the `export` builtin results in a pipeline expansion error

Reproduction:

# alias foo="export FOO='bar'"
# foo

Expected behaviour: executes export FOO='bar'.

Actual behaviour: fails with error:

ion: pipeline expansion error: unable to pipe inputs to alias: 'foo = export FOO='bar''

Build information:

# uname -a
Linux a 4.19.30_1 #1 SMP PREEMPT Wed Mar 20 09:17:21 UTC 2019 x86_64 GNU/Linux
# ion --version
ion 1.0.0-alpha (x86_64-unknown-linux-gnu)
rev fde4aff113a0c7690a158d55ff1798b6d1e91369

ion was built with rustc 1.31.0