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('should normalize props with default values', () => {
    var Component = React.createClass({
      getDefaultProps: function() {
        return {prop: 'testKey'};
      },
      render: function() {
        return React.createElement('span', null, this.props.prop);
      },
    });
    var instance = ReactTestUtils.renderIntoDocument(
      React.createElement(Component)
    );
    expect(instance.props.prop).toBe('testKey');
    var inst2 = ReactTestUtils.renderIntoDocument(
      React.createElement(Component, {prop: null})
    );
    expect(inst2.props.prop).toBe(null);
  });
  it('throws when changing a prop (in dev) after element creation', () => {
    var Outer = React.createClass({
      render: function() {
        var el = <div className="moo" />;
        expect(function() {
          el.props.className = 'quack';
        }).toThrow();
        expect(el.props.className).toBe('moo');
        return el;
      },
    });
    var outer = ReactTestUtils.renderIntoDocument(<Outer color="orange" />);