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`?

return (
      typeof prototype.render === 'function' &&
      typeof prototype.setState === 'function'
    );
  },
  isCompositeComponentElementWithType: function(inst, type) {
    var internalInstance = ReactInstanceMap.get(inst);
    var constructor = internalInstance
      ._currentElement
      .type;
    return !!(ReactTestUtils.isCompositeComponentElement(inst) &&
             (constructor === type));
  },
  getRenderedChildOfCompositeComponent: function(inst) {
    if (!ReactTestUtils.isCompositeComponent(inst)) {
      return null;
    }
    var internalInstance = ReactInstanceMap.get(inst);
    return internalInstance._renderedComponent.getPublicInstance();
  },
  findAllInRenderedTree: function(inst, test) {
    if (!inst) {
      return [];
    }
    invariant(
      ReactTestUtils.isCompositeComponent(inst),
      'findAllInRenderedTree(...): instance must be a composite component'
    );
    return findAllInRenderedTreeInternal(ReactInstanceMap.get(inst), test);
  },