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(element.props).toEqual({foo: '56'});
  });
  it('extracts null key and ref', () => {
    var element = React.createFactory(ComponentClass)({
      key: null,
      ref: null,
      foo: '12',
    });
    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({foo: '12'});
  });
  it('ignores undefined key and ref', () => {
    var props = {
      foo: '56',
      key: undefined,
      ref: undefined,
    };
    var element = React.createFactory(ComponentClass)(props);
    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({foo: '56'});
  });
  it('ignores key and ref warning getters', () => {