Parsing error under certain scenarios when using async as shorthand property in literal

[DrewML]

Originally reported by @Blesh in babel/babel#4384 - opening an issue here because it looks like a Babylon bug.

Test Case

babylon.parse('var foo = {async, bar}') // SyntaxError: Unexpected token (1:16)

As pointed out by Ben, this failure doesn't happen in some other situations

Working examples

babylon.parse('var foo = {async}')
babylon.parse('var foo = {bar, async}')
babylon.parse('var foo = {async: async, bar}')

[DrewML]

I have this fixed locally. PR coming soon, once I figure out the issue I am having with adding a new test case.