Array.from polyfill

Array.from polyfill
I don't see a test for Array subclass.

Here is some code to get you started (not tested).

```js
var length = 0;
var constructorCounter = 0;
var lengthSetCounter = 0;
var lengthGetCounter = 0;

class MyArray extends Array {
  constructor(v) {
    constructorCounter++:
    assert.equal(v, 2);
  }
  set length(v) {
    lengthSetCounter++;
    length = v;
  }
  get length() {
    return length;
  }
}

var ma = MyArray.from(['a', 'b']);
assert.instanceOf(ma, MyArray);
assert.equal(constructorCounter,1);
assert.equal(lengthGetCounter, 0);
assert.equal(lengthSetCounter, 1);
assert.equal(length, 2);
assert.isTrue(ma.hasOwnProperty('0'));
assert.isTrue(ma.hasOwnProperty('1'));
assert.isFalse(ma.hasOwnProperty('length'));
assert.equal(ma[0], 'a');
assert.equal(ma[1], 'b');
assert.equal(ma.length, 2);
```