Rather minor issue, but the fix should be simple enough so I thought that I'd file this anyway:

When using `react-addons-testutils`, sometimes I write simple errors such as 

```
TestUtils.scryRenderedDOMComponentsWithClass("foo")

// expected:
TestUtils.scryRenderedDOMComponentsWithClass(reactComponentInstance, "foo")

```

This throws a stack trace of ~80 lines with a type error of `TypeError: Cannot read property 'split' of undefined`. Fixing this is trivial (and the first line of the stack trace points to the offending line as expected), but the error itself isn't really obvious. 

Some others such as `scryRenderedDOMComponentsWithTag` don't throw until `findAllInRenderedTree` is reached, which throws a bit more helpful error of `Error: findAllInRenderedTree(...): instance must be a composite component(…)`. 

Would it make sense to check that the arguments passed are valid and throw immediately if they aren't, so the user would end up with errors along the lines of `scryRenderedDOMComponentsWithClass: first argument must be an instance of ReactComponent`?

/**
 * @class ReactTestUtils
 */
function findAllInRenderedTreeInternal(inst, test) {