Reaches parse error with async functions

[Karolusrex]

Repro:

example.js:

!function (){}()
async function runIt(fun){ fun() }
runIt(function (){})

optimize-js www/example.js

{
 SyntaxError: Unexpected token (2:6)
    at Parser.pp$4.raise (/Users/lundfall/.nvm/versions/node/v7.10.0/lib/node_modules/optimize-js/node_modules/acorn/dist/acorn.js:2223:15)
    at Parser.pp.unexpected (/Users/lundfall/.nvm/versions/node/v7.10.0/lib/node_modules/optimize-js/node_modules/acorn/dist/acorn.js:605:10)
    at Parser.pp.semicolon (/Users/lundfall/.nvm/versions/node/v7.10.0/lib/node_modules/optimize-js/node_modules/acorn/dist/acorn.js:583:61)
    at Parser.pp$1.parseExpressionStatement (/Users/lundfall/.nvm/versions/node/v7.10.0/lib/node_modules/optimize-js/node_modules/acorn/dist/acorn.js:968:10)
    at Parser.pp$1.parseStatement (/Users/lundfall/.nvm/versions/node/v7.10.0/lib/node_modules/optimize-js/node_modules/acorn/dist/acorn.js:732:24)
    at Parser.pp$1.parseTopLevel (/Users/lundfall/.nvm/versions/node/v7.10.0/lib/node_modules/optimize-js/node_modules/acorn/dist/acorn.js:640:25)
    at Parser.parse (/Users/lundfall/.nvm/versions/node/v7.10.0/lib/node_modules/optimize-js/node_modules/acorn/dist/acorn.js:518:17)
    at Object.parse (/Users/lundfall/.nvm/versions/node/v7.10.0/lib/node_modules/optimize-js/node_modules/acorn/dist/acorn.js:3100:39)
    at optimizeJs (/Users/lundfall/.nvm/versions/node/v7.10.0/lib/node_modules/optimize-js/lib/index.js:9:19)
    at /Users/lundfall/.nvm/versions/node/v7.10.0/lib/node_modules/optimize-js/lib/bin.js:26:15 pos: 23, loc: Position { line: 2, column: 6 }, raisedAt: 31 }