SVG elements aren't polyfilled

[treshugart]

Test to reproduce:

describe('SVGElement', function () {
  it('should be polyfilled', function () {
    const div = document.createElement('div');
    const svg = document.createElementNS('http://www.w3.org/2000/svg', 'svg');
    div.attachShadow({ mode: 'open' });
    div.shadowRoot.appendChild(document.createElement('slot'));
    div.appendChild(svg);
    expect(svg.parentNode).to.equal(div);
  });
});

We probably don't need to test all methods and properties like we do on HTMLElement, but we should ensure that the polyfilled methods on SVGElement equal the methods on HTMLElement.