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.

expect(ReactDOM.findDOMNode(outer).className).toBe('moo');
  });
  it('throws when adding a prop (in dev) after element creation', () => {
    var container = document.createElement('div');
    var Outer = React.createClass({
      getDefaultProps: () => ({sound: 'meow'}),
      render: function() {
        var el = <div>{this.props.sound}</div>;
        expect(function() {
          el.props.className = 'quack';
        }).toThrow();
        expect(el.props.className).toBe(undefined);
        return el;
      },
    });
    var outer = ReactDOM.render(<Outer />, container);
    expect(ReactDOM.findDOMNode(outer).textContent).toBe('meow');
    expect(ReactDOM.findDOMNode(outer).className).toBe('');
  });
  it('does not warn for NaN props', () => {
    spyOn(console, 'error');
    var Test = React.createClass({
      render: function() {
        return <div />;
      },
    });
    var test = ReactTestUtils.renderIntoDocument(<Test value={+undefined} />);
    expect(test.props.value).toBeNaN();
    expect(console.error.calls.count()).toBe(0);
  });