Inspired by this question Using third-party libraries - always use a wrapper? I wanted to know what people actually consider as third-party libraries.

Example from PHP:
If I'm building an application using Zend framework, should I treat Zend framework libraries as third party code?

Example from C#:
If I'm building a desktop application, should I treat all .Net classes as third party code?

Example from Java:
Should I treat all libraries in the JDK as third party libraries?

Some people say that if a library is stable and won't change often then one doesn't need to wrap it. However I fail to see how one would test a class that depends on a third party code without wrapping it.