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`?

}
/**
 * Todo: Support the entire DOM.scry query syntax. For now, these simple
 * utilities will suffice for testing purposes.
 * @lends ReactTestUtils
 */
var ReactTestUtils = {
  renderIntoDocument: function(instance) {
    var div = document.createElement('div');
    
    
    
    
    
    return ReactDOM.render(instance, div);
  },
  isElement: function(element) {
    return ReactElement.isValidElement(element);
  },
  isElementOfType: function(inst, convenienceConstructor) {
    return (
      ReactElement.isValidElement(inst) &&
      inst.type === convenienceConstructor
    );
  },
  isDOMComponent: function(inst) {
    return !!(inst && inst.nodeType === 1 && inst.tagName);
  },
  isDOMComponentElement: function(inst) {
    return !!(inst &&