Returns the key value for the node of the current wrapper.
NOTE: can only be called on a wrapper of a single node.
const wrapper = mount(
<ul>
{['foo', 'bar'].map(s => <li key={s}>{s}</li>)}
</ul>
).find('li');
expect(wrapper.at(0).key()).to.equal('foo');
expect(wrapper.at(1).key()).to.equal('bar');