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 this;
  },
  /**
   * Simulates a top level event being dispatched from a raw event that occurred
   * on an `Element` node.
   * @param {Object} topLevelType A type from `EventConstants.topLevelTypes`
   * @param {!Element} node The dom to simulate an event occurring on.
   * @param {?Event} fakeNativeEvent Fake native event to use in SyntheticEvent.
   */
  simulateNativeEventOnNode: function(topLevelType, node, fakeNativeEvent) {
    fakeNativeEvent.target = node;
    ReactBrowserEventEmitter.ReactEventListener.dispatchEvent(
      topLevelType,
      fakeNativeEvent
    );
  },
  /**
   * Simulates a top level event being dispatched from a raw event that occurred
   * on the `ReactDOMComponent` `comp`.
   * @param {Object} topLevelType A type from `EventConstants.topLevelTypes`.
   * @param {!ReactDOMComponent} comp
   * @param {?Event} fakeNativeEvent Fake native event to use in SyntheticEvent.
   */
  simulateNativeEventOnDOMComponent: function(
      topLevelType,
      comp,
      fakeNativeEvent) {
    ReactTestUtils.simulateNativeEventOnNode(
      topLevelType,