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`?

}
};
/**
 * Exports:
 *
 * - `ReactTestUtils.Simulate.click(Element/ReactDOMComponent)`
 * - `ReactTestUtils.Simulate.mouseMove(Element/ReactDOMComponent)`
 * - `ReactTestUtils.Simulate.change(Element/ReactDOMComponent)`
 * - ... (All keys from event plugin `eventTypes` objects)
 */
function makeSimulator(eventType) {
  return function(domComponentOrNode, eventData) {
    var node;
    invariant(
      !React.isValidElement(domComponentOrNode),
      'TestUtils.Simulate expects a component instance and not a ReactElement.' +
      'TestUtils.Simulate will not work if you are using shallow rendering.'
    );
    if (ReactTestUtils.isDOMComponent(domComponentOrNode)) {
      node = findDOMNode(domComponentOrNode);
    } else if (domComponentOrNode.tagName) {
      node = domComponentOrNode;
    }
    var dispatchConfig =
      EventPluginRegistry.eventNameDispatchConfigs[eventType];
    var fakeNativeEvent = new Event();
    fakeNativeEvent.target = node;
    
    
    var event = new SyntheticEvent(