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(console.error.calls.count()).toBe(1);
    expect(console.error.calls.argsFor(0)[0]).toContain(
      'React.cloneElement(...): Expected props argument to be a plain object. ' +
      'Properties defined in its prototype chain will be ignored.'
    );
  });
  it('does not fail if config has no prototype', function() {
    var config = Object.create(null, {foo: {value: 1, enumerable: true}});
    React.cloneElement(<div />, config);
  });
  it('should keep the original ref if it is not overridden', function() {
    var Grandparent = React.createClass({
      render: function() {
        return <Parent child={<div ref="yolo" />} />;
      },
    });
    var Parent = React.createClass({
      render: function() {
        return (
          <div>
            {React.cloneElement(this.props.child, { className: 'xyz' })}
          </div>
        );
      },
    });
    var component = ReactTestUtils.renderIntoDocument(<Grandparent />);
    expect(component.refs.yolo.tagName).toBe('DIV');
  });
  it('should transfer the key property', function() {
    var Component = React.createClass({