There is a bug in the logic for determining if `props` is a plain object. This bug results in a false positive, flooding the console with hundreds of instances of this warning: "Expected props argument to be a plain object. Properties defined in its prototype chain will be ignored.".

The code that's failing is in [ReactElement.js, Line 210 in the v15.3.1 branch](https://github.com/facebook/react/blob/v15.3.1/src/isomorphic/classic/element/ReactElement.js#L210) (line number is probably different in master)

> config.**proto** == null || config.**proto** === Object.prototype,

This fails when the props object is created in one iframe, but uses an instance of React from another frame. Each browser window/tab/frame has it's own instance of the Object constructor. For instance, `document.querySelector('iframe').contentWindow.Object === Object` always results in `false`.

Check out this [demo](https://jsfiddle.net/05ugxo5f/4/), which shows the false-positive warning message.

it('should clone a DOM component with new props', () => {
    var Grandparent = React.createClass({
      render: function() {
        return <Parent child={<div className="child" />} />;
      },
    });
    var Parent = React.createClass({
      render: function() {
        return (
          <div className="parent">
            {React.cloneElement(this.props.child, { className: 'xyz' })}
          </div>
        );
      },
    });
    var component = ReactTestUtils.renderIntoDocument(<Grandparent />);
    expect(ReactDOM.findDOMNode(component).childNodes[0].className).toBe('xyz');
  });
  it('should clone a composite component with new props', () => {
    var Child = React.createClass({
      render: function() {
        return <div className={this.props.className} />;
      },
    });
    var Grandparent = React.createClass({
      render: function() {
        return <Parent child={<Child className="child" />} />;
      },
    });
    var Parent = React.createClass({