JS: Fix flatMap thisArg flowing to the callback index

[stonechoe]

The Array#flatMap summary sends thisArg to the wrong callback parameter. It sends thisArg to the index parameter (Parameter[1]). The correct target is the this value of the callback (Parameter[this]).

The summaries for map, filter, find, every and some already use Parameter[this]. This change makes flatMap do the same.

[0].flatMap(function (x, i) {
  sink(this); // Before: no flow. After: flow.
  sink(i);    // Before: flow. After: no flow.
  return [];
}, source());

Specification: Array.prototype.flatMap

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[asgerf]

Thanks for the PR! Looks good to me.