Looking from outside both Symfony2 and ZF2 is full of standalone components. But the reality is not the same. Though Symfony2 components are split into each components in github, you cannot give a pull request to that component. The tests for all the components still resides in the core. The same will be applying to ZF2 too. I wrote my concern on the topic in mailing list of Symony2, if you are interested you can read on it here.
Let's leave the contributions or pull requests, if I / you are trying to integrate Symfony form component to your project or library, we want to bring the tests some how make the necessary changes to the core if I want to make use of Event Dispatcher / Signal or another like Aura or Zend. Try to make how you can render in your view if I am using Aura View or Zend or even in phly_mustache.
So let me tell you the design principle to make them as standalone have some failure. Coming back to Aura, as a small contributor I can see Paul M Jones right decision to make the component library has more to speak. The good thing is Aura has all the tests in one place for the component. For eg: consider Aura Router the routing component. The source lies in src folder , the tests in tests folder.
My biases in posting this should be clear. ;-) Via Is there a design flaw for the Components or Packages made by Symfony2 and ZF2 | harikt.com.