UnderlineNav fixes

[shawnbot]

This addresses a couple of points in #128:

1. Removes a circular import between UnderlineNav and UnderlineNavLink by using duck typing to detect UnderlineNavLink instances in UnderlineNav.
2. Moves the rendersClass() testing helper into utils/testing.

I also got a change to package-lock.json when I ran npm install, so I've included that in this PR. I can remove that commit if there's a problem with it, though.

[shawnbot]

This could probably just be:

newProps.className = className
return React.cloneElement(child, newProps)

[emplums]

I think the shorthand here is fine!

[shawnbot]

I have one minor concern about the duck typing approach of testing child.type.name, which should be the name of the component function or class. It's possible that code minifying tools like Uglify can rename these functions to have shorter names unless you tell them not to "mangle" them. For instance, this:

export default function MyComponent(props) {
  return <div>{props.children}</div>
}

might be minified with mangled function names into:

exports=function z(b){return A.createElement('div',{},b.children)}

...in which case the export's name will be z, not MyComponent. Various React tools understand this and the conventional solution is to set the displayName property on a component (function or class), which minifiers won't touch. Unfortunately, it looks like NavLink doesn't set displayName, so we can't use that to duck type.

Anyway, it works for now, and you can see in the live docs that it works in our production bundle, but it's something to be aware of.

[emplums]

Looks good to me! I will keep the class name duck typing in mind when looking through the emotion/system-classname stuff. Not sure if emotion minifies the class names or not!

[shawnbot]

Good call, @emplums. We might want to set displayName on our components just to be safe.