Support computed properties downlevel

[JsonFreeman]

We can emit computed properties in ES5, using the following scheme:

var x = {
    a: 0,
    ["b"]: 0,
    ["c"]() {},
    get ["d"]() { return 0 },
    set ["d"](x) {},
    e: 0
};

emits as:

var x = {
    a: 0
};
x["b"]: 0;
x["c"] = function() {};
Object.defineProperty(x, "d" ...);
Object.defineProperty(x, "d" ...);
x.e = 0;

In any other expression context, we would emit a temp variable like we do for destructuring.

[DanielRosenwasser]

I dunno about that emit - what about this?

var x = { ["abc"]: 10 },
    y = (x = undefined, 10);

I think we'll have to do something like the following:

var x = (x = { }, x["abc"] = 10, x),
    y = (x = undefined, 10);

[JsonFreeman]

Yeah, when there are multiple variable declarations in the same variable statement, we would have to do that.