Avoid deoptimization in generateCSS()

[lencioni]

I was profiling the css() function and Chrome raised a flag on this
function:

Not optimized: Bad value context for arguments value

More info on this warning:
GoogleChrome/devtools-docs#53 (comment)

Looking at the warning and the compiled version of this code, it seems
to do some things with arguments when using the default values
here, which is causing this deoptimization.

var selectorHandlers = arguments.length <= 2 || arguments[2] === undefined ? [] : arguments[2];
var stringHandlers = arguments.length <= 3 || arguments[3] === undefined ? {} : arguments[3];
var useImportant = arguments.length <= 4 || arguments[4] === undefined ? true : arguments[4];

By removing the default values for the arguments, the deoptimization
disappears. I thought about adding logic that would provide values for
these arguments if they aren't defined, but since the only thing that
relies on that is tests I decided to just update the tests to always
pass all of the arguments.

In my benchmark, this does not seem to make much of a difference but it
still seems like a good idea to avoid things that the browser tells us
is deoptimized.

[xymostech]

Interesting! Changes LGTM. Does this mean that we shouldn't use default argument values anywhere?

[lencioni]

Possibly, or maybe the Babel transform can be modified in a way that avoids this? In any case, it was hard for me to see what the real world performance impact of this actually is. Maybe @twokul or @paulirish can point us in the right direction here?