I believe #5744 introduced a behavioral difference between development and production versions of React. We released it as a part of 15.0, and this difference still exists.

The production behavior hasn’t changed. However the development behavior diverged after this change.

``` js
var el = React.createElement('div', { key: undefined })
document.body.innerHTML = (typeof el.key) + ' ' + el.key
```

What do you expect to appear?
### 0.14.x
- Development (https://jsfiddle.net/sjLcmnq6/1/): `object null`
- Production (https://jsfiddle.net/qro1wck0/1/): `object null`
## 15.x
- Development (https://jsfiddle.net/69z2wepo/43538/):  **`string undefined`**
- Production (https://jsfiddle.net/rakg6uwy/1/): `object null`

The 15.x dev behavior is different because [dev code path only checks the existence of a property but not whether it’s undefined](https://github.com/alitaheri/react/blob/c3980a6d08257d39a08bed8b116d53a56344166b/src/isomorphic/classic/element/ReactElement.js#L130-L133) whereas the [prod code path checks for undefined explicitly](https://github.com/alitaheri/react/blob/c3980a6d08257d39a08bed8b116d53a56344166b/src/isomorphic/classic/element/ReactElement.js#L135-L136).

`React.cloneElement()` is not affected because it doesn’t include those warnings.

---

This doesn’t seem like a major problem (nobody noticed it!) in the real world, but I think we should fix this.
I have a related fix in the works so I’ll add some more tests and include it.

expect(element.props.children).toBe(undefined);
    var element2 = React.cloneElement(React.createElement(ComponentClass, {
      children: 'text',
    }), {}, undefined);
    expect(element2.props.children).toBe(undefined);
  });
  it('merges rest arguments onto the children prop in an array', function() {
    spyOn(console, 'error');
    var a = 1;
    var b = 2;
    var c = 3;
    var element = React.createFactory(ComponentClass)(null, a, b, c);
    expect(element.props.children).toEqual([1, 2, 3]);
    expect(console.error.calls.count()).toBe(0);
  });
  
  
  it('allows static methods to be called using the type property', function() {
    spyOn(console, 'error');
    var StaticMethodComponentClass = React.createClass({
      statics: {
        someStaticMethod: function() {
          return 'someReturnValue';
        },
      },
      getInitialState: function() {
        return {valueToReturn: 'hi'};
      },
      render: function() {
        return React.createElement('div');