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`?

}
var oldInjectEventPluginOrder = EventPluginHub.injection.injectEventPluginOrder;
EventPluginHub.injection.injectEventPluginOrder = function() {
  oldInjectEventPluginOrder.apply(this, arguments);
  buildSimulators();
};
var oldInjectEventPlugins = EventPluginHub.injection.injectEventPluginsByName;
EventPluginHub.injection.injectEventPluginsByName = function() {
  oldInjectEventPlugins.apply(this, arguments);
  buildSimulators();
};
buildSimulators();
/**
 * Exports:
 *
 * - `ReactTestUtils.SimulateNative.click(Element/ReactDOMComponent)`
 * - `ReactTestUtils.SimulateNative.mouseMove(Element/ReactDOMComponent)`
 * - `ReactTestUtils.SimulateNative.mouseIn/ReactDOMComponent)`
 * - `ReactTestUtils.SimulateNative.mouseOut(Element/ReactDOMComponent)`
 * - ... (All keys from `EventConstants.topLevelTypes`)
 *
 * Note: Top level event types are a subset of the entire set of handler types
 * (which include a broader set of "synthetic" events). For example, onDragDone
 * is a synthetic event. Except when testing an event plugin or React's event
 * handling code specifically, you probably want to use ReactTestUtils.Simulate
 * to dispatch synthetic events.
 */
function makeNativeSimulator(eventType) {
  return function(domComponentOrNode, nativeEventData) {
    var fakeNativeEvent = new Event(eventType);