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.

global.Symbol = undefined;
    React = require('React');
    ReactDOM = require('ReactDOM');
    ReactTestUtils = require('ReactTestUtils');
    
    
    ComponentClass = React.createClass({
      render: function() {
        return React.createElement('div');
      },
    });
  });
  afterEach(() => {
    global.Symbol = originalSymbol;
  });
  it('uses the fallback value when in an environment without Symbol', () => {
    expect(<div />.$$typeof).toBe(0xeac7);
  });
  it('returns a complete element according to spec', () => {
    var element = React.createFactory(ComponentClass)();
    expect(element.type).toBe(ComponentClass);
    expect(element.key).toBe(null);
    expect(element.ref).toBe(null);
    expect(Object.isFrozen(element)).toBe(true);
    expect(Object.isFrozen(element.props)).toBe(true);
    expect(element.props).toEqual({});
  });
  it('should warn when `key` is being accessed on createClass element', () => {
    spyOn(console, 'error');
    var container = document.createElement('div');