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`?

._renderValidatedComponentWithoutOwnerOrContext,
  }
);
ReactShallowRenderer.prototype.render = function(element, context) {
  
  
  
  ReactDefaultInjection.inject();
  invariant(
    ReactElement.isValidElement(element),
    'ReactShallowRenderer render(): Invalid component element.%s',
    typeof element === 'function' ?
      ' Instead of passing a component class, make sure to instantiate ' +
      'it by passing it to React.createElement.' :
      ''
  );
  invariant(
    typeof element.type !== 'string',
    'ReactShallowRenderer render(): Shallow rendering works only with custom ' +
    'components, not primitives (%s). Instead of calling `.render(el)` and ' +
    'inspecting the rendered output, look at `el.props` directly instead.',
    element.type
  );
  if (!context) {
    context = emptyObject;
  }
  ReactUpdates.batchedUpdates(_batchedRender, this, element, context);
  return this.getRenderOutput();
};
function _batchedRender(renderer, element, context) {