Js parser fails to parse arrow functions in for loop defintion.

[spotandjake]

Describe the bug

Trying to link to a JS file which contains an arrow function in the variable definition of a for loop fails to parse. Such as the example below.

for (var t=()=>1; false; undefined) {
  console.log("test");
}

Expected behaviour
This should parse and be handled as valid js.

Versions
js_of_ocaml: 6.2.0

Additional information

I checked the parser and it doesn't seem to handle this at all however it's valid js and correctly parsed by node.js. It is certainly weird js however the case where I ran into this bug was trying to get an es6 package to work in libbinaryen.