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`?

/**
 * Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule ReactTestUtils
 */
'use strict';
var EventConstants = require('EventConstants');
var EventPluginHub = require('EventPluginHub');
var EventPluginRegistry = require('EventPluginRegistry');
var EventPropagators = require('EventPropagators');
var React = require('React');
var ReactDefaultInjection = require('ReactDefaultInjection');
var ReactDOM = require('ReactDOM');
var ReactDOMComponentTree = require('ReactDOMComponentTree');
var ReactElement = require('ReactElement');
var ReactBrowserEventEmitter = require('ReactBrowserEventEmitter');
var ReactCompositeComponent = require('ReactCompositeComponent');
var ReactInstanceMap = require('ReactInstanceMap');
var ReactUpdates = require('ReactUpdates');
var SyntheticEvent = require('SyntheticEvent');
var emptyObject = require('emptyObject');
var findDOMNode = require('findDOMNode');
var invariant = require('invariant');
var topLevelTypes = EventConstants.topLevelTypes;
function Event(suffix) {}