Re: [ServiceWorker] window.caches and window.fetch should be [Replaceable] (#535)

@jakearchibald this is your brain on readonly no-[Replacable]:

```js
class Foo {
    constructor() {
        this._bar = 5;
    }

    get bar() {
        return this._bar;
    }
}
```

This is your brain on readonly [Replacable]:

```js
class Foo {
    constructor() {
        this._bar = 5;
    }

    get bar() {
        return this._bar;
    }

    set bar(v) {
        Object.defineProperty(this, "bar", {
            writable: true,
            enumerable: true,
            configurable: true,
            value: v
        });
    }
}
```

For comparison this is non-readonly (and thus non-[Replacable])

```js
class Foo {
    constructor() {
        this._bar = 5;
    }

    get bar() {
        return this._bar;
    }

    set bar(v) {
        this._bar = v;
    }
}
```

See also the example at https://heycam.github.io/webidl/#Replaceable

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https://github.com/slightlyoff/ServiceWorker/issues/535#issuecomment-65843874