Fix arrow function lowering failing to emit var _this decl

[nkeynes]

Fixes issue where the synthetic _this variable was being dropped, so that:

function foo(a) { 
  arr.forEach(e=>this.foo(e));
}

was incorrectly lowered to the nonfunctional code:

function foo(a) {
  arr.forEach(function(e) {
    return _this.foo(e);
  });
}

[evanw]

Thanks for the fix. This code clearly should have been covered by tests. Sorry about that.

A note for myself: Looks like this was introduced in version 0.7.2 (specifically in this commit) and regressed recently in version 0.14.30 (specifically in this commit).