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`?

findDOMNode(comp),
      fakeNativeEvent
    );
  },
  nativeTouchData: function(x, y) {
    return {
      touches: [
        {pageX: x, pageY: y},
      ],
    };
  },
  createRenderer: function() {
    return new ReactShallowRenderer();
  },
  Simulate: null,
  SimulateNative: {},
};
/**
 * @class ReactShallowRenderer
 */
var ReactShallowRenderer = function() {
  this._instance = null;
};
ReactShallowRenderer.prototype.getMountedInstance = function() {
  return this._instance ? this._instance._instance : null;
};
var NoopInternalComponent = function(element) {
  this._renderedOutput = element;
  this._currentElement = element;
};