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('identifies elements, but not JSON, if Symbols are supported', () => {
    
    
    
    var REACT_ELEMENT_TYPE = function() {}; 
    var OTHER_SYMBOL = function() {}; 
    global.Symbol = function(name) {
      return OTHER_SYMBOL;
    };
    global.Symbol.for = function(key) {
      if (key === 'react.element') {
        return REACT_ELEMENT_TYPE;
      }
      return OTHER_SYMBOL;
    };
    jest.resetModuleRegistry();
    React = require('React');
    var Component = React.createClass({
      render: function() {
        return React.createElement('div');
      },
    });
    expect(React.isValidElement(React.createElement('div')))
      .toEqual(true);
    expect(React.isValidElement(React.createElement(Component)))
      .toEqual(true);
    expect(React.isValidElement(null)).toEqual(false);
    expect(React.isValidElement(true)).toEqual(false);