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`?

findRenderedDOMComponentWithClass: function(root, className) {
    var all =
      ReactTestUtils.scryRenderedDOMComponentsWithClass(root, className);
    if (all.length !== 1) {
      throw new Error(
        'Did not find exactly one match (found: ' + all.length + ') ' +
        'for class:' + className
      );
    }
    return all[0];
  },
  /**
   * Finds all instance of components in the rendered tree that are DOM
   * components with the tag name matching `tagName`.
   * @return {array} an array of all the matches.
   */
  scryRenderedDOMComponentsWithTag: function(root, tagName) {
    return ReactTestUtils.findAllInRenderedTree(root, function(inst) {
      return ReactTestUtils.isDOMComponent(inst) &&
            inst.tagName.toUpperCase() === tagName.toUpperCase();
    });
  },
  /**
   * Like scryRenderedDOMComponentsWithTag but expects there to be one result,
   * and returns that one result, or throws exception if there is any other
   * number of matches besides one.
   * @return {!ReactDOMComponent} The one match.
   */
  findRenderedDOMComponentWithTag: function(root, tagName) {
    var all = ReactTestUtils.scryRenderedDOMComponentsWithTag(root, tagName);