"Illegal invocation" when using toString() on a cy.spy for a native object

[flotwig]

Describe the bug

Trying to run toString on certain spies results in an error.

In Chromium-based browsers:

TypeError: Illegal invocation
    at Function.toString (sinon.js:3417)
    at JjogzZG?editors=1000:26

In Firefox:

TypeError: "'get onclick' called on an object that does not implement interface Notification."
    sinon 8.1.1/pkg/sinon.js:3417
    <anonymous> https://cdpn.io/boomboom/v2/index.html?editors=1000&key=iFrameKey-ce3ac7bf-f321-9ff3-df5c-2db10a31e480:22

To Reproduce

On line 3, the error will be thrown:

const spy = sinon.spy(window, 'Notification')
new Notification('hello')
console.log(spy.toString())

CodePen (check console for output): https://codepen.io/flotwig/pen/JjogzZG?editors=1000

Expected behavior

toString works without erroring.

Context (please complete the following information):

• Library version: 8.1.1
• Environment: Firefox/Chromium-based
• Example URL: https://codepen.io/flotwig/pen/JjogzZG?editors=1000
• Other libraries you are using: n/a

[flotwig]

Opened a PR with a fix that should be suitable: #2216