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`?

/**
   * Finds all instance of components in the rendered tree that are DOM
   * components with the class name matching `className`.
   * @return {array} an array of all the matches.
   */
  scryRenderedDOMComponentsWithClass: function(root, classNames) {
    if (!Array.isArray(classNames)) {
      classNames = classNames.split(/\s+/);
    }
    return ReactTestUtils.findAllInRenderedTree(root, function(inst) {
      if (ReactTestUtils.isDOMComponent(inst)) {
        var className = inst.className;
        if (typeof className !== 'string') {
          
          className = inst.getAttribute('class') || '';
        }
        var classList = className.split(/\s+/);
        return classNames.every(function(name) {
          return classList.indexOf(name) !== -1;
        });
      }
      return false;
    });
  },
  /**
   * Like scryRenderedDOMComponentsWithClass 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.
   */