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('merges an additional argument onto the children prop', () => {
    spyOn(console, 'error');
    var a = 1;
    var element = React.createFactory(ComponentClass)({
      children: 'text',
    }, a);
    expect(element.props.children).toBe(a);
    expect(console.error.calls.count()).toBe(0);
  });
  it('does not override children if no rest args are provided', () => {
    spyOn(console, 'error');
    var element = React.createFactory(ComponentClass)({
      children: 'text',
    });
    expect(element.props.children).toBe('text');
    expect(console.error.calls.count()).toBe(0);
  });
  it('overrides children if null is provided as an argument', () => {
    spyOn(console, 'error');
    var element = React.createFactory(ComponentClass)({
      children: 'text',
    }, null);
    expect(element.props.children).toBe(null);
    expect(console.error.calls.count()).toBe(0);
  });
  it('merges rest arguments onto the children prop in an array', () => {
    spyOn(console, 'error');
    var a = 1;
    var b = 2;