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Should be self-explanatory.

Honestly, I'm not finding it that easy to see what's going on there :) So you've got promises as map keys?

Here's a bodged up Map implementation:

function Map(keyVal) {
  this._keys = [];
  this._values = [];
  if (keyVal) keyVal.forEach(k=>{
    this._keys.push(k[0]);
    this._values.push(k[1]);
  });
  this.size = this._keys.length;
}
Map.prototype.has = function(k) {
  return this._keys.indexOf(k)>=0;
};
Map.prototype.get = function(k) {
  var i = this._keys.indexOf(k);
  if (i<0) return;
  return this._values[i];
};
Map.prototype.set = function(k,v) {
  var i = this._keys.indexOf(k);
  if (i<0) {
    this._keys.push(k);
    this._values.push(v);
    this.size = this._keys.length;
  } else this._values[i] = v;
  return this;
};
Map.prototype.delete = function(k) {
  var i = this._keys.indexOf(k);
  if (i<0) return false;
  this._keys.splice(i,1);
  this._values.splice(i,1);
  this.size = this._keys.length;
};
Map.prototype.keys = function() { return this._keys; };
Map.prototype.values = function() { return this._values; };

It's actually pretty quick because it uses Array.indexOf. It doesn't work for NaN (that could be special-cased) but I think it works for most other stuff.

I've added it to the ES6 wishlist linked above, but looking at the API I wonder if it makes much sense without Iterator support?