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('does not warn when the element is directly in rest args', () => {
    spyOn(console, 'error');
    React.cloneElement(<div />, null, <div />, <div />);
    expect(console.error.calls.count()).toBe(0);
  });
  it('does not warn when the array contains a non-element', () => {
    spyOn(console, 'error');
    React.cloneElement(<div />, null, [{}, {}]);
    expect(console.error.calls.count()).toBe(0);
  });
  it('should check declared prop types after clone', () => {
    spyOn(console, 'error');
    var Component = React.createClass({
      propTypes: {
        color: React.PropTypes.string.isRequired,
      },
      render: function() {
        return React.createElement('div', null, 'My color is ' + this.color);
      },
    });
    var Parent = React.createClass({
      render: function() {
        return React.cloneElement(this.props.child, {color: 123});
      },
    });
    var GrandParent = React.createClass({
      render: function() {
        return React.createElement(
          Parent,
          { child: React.createElement(Component, {color: 'red'}) }