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.

ReactTestUtils.renderIntoDocument(
      React.cloneElement(<Component />, {children: 'xyz'})
    );
  });
  it('should shallow clone children', () => {
    var Component = React.createClass({
      render: function() {
        expect(this.props.children).toBe('xyz');
        return <div />;
      },
    });
    ReactTestUtils.renderIntoDocument(
      React.cloneElement(<Component>xyz</Component>, {})
    );
  });
  it('should accept children as rest arguments', () => {
    var Component = React.createClass({
      render: function() {
        return null;
      },
    });
    var clone = React.cloneElement(
      <Component>xyz</Component>,
      { children: <Component /> },
      <div />,
      <span />
    );
    expect(clone.props.children).toEqual([
      <div />,
      <span />,