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.

var Child = React.createClass({
      render: function() {
        return <div> {this.props.key} </div>;
      },
    });
    var Parent = React.createClass({
      render: function() {
        return (
          <div>
            <Child key="0" />
            <Child key="1" />
            <Child key="2" />
          </div>
        );
      },
    });
    expect(console.error.calls.count()).toBe(0);
    ReactDOM.render(<Parent />, container);
    expect(console.error.calls.count()).toBe(1);
    expect(console.error.calls.argsFor(0)[0]).toContain(
      'Child: `key` is not a prop. Trying to access it will result ' +
      'in `undefined` being returned. If you need to access the same ' +
      'value within the child component, you should pass it as a different ' +
      'prop. (https://fb.me/react-special-props)'
    );
  });
  it('should warn when `key` is being accessed on ES class element', () => {
    spyOn(console, 'error');
    var container = document.createElement('div');
    class Child extends React.Component {