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.

if (__DEV__) {
    if (hasOwnProperty.call(config, 'ref')) {
      var getter = Object.getOwnPropertyDescriptor(config, 'ref').get;
      if (getter && getter.isReactWarning) {
        return false;
      }
    }
  }
  return config.ref !== undefined;
}
function hasValidKey(config) {
  if (__DEV__) {
    if (hasOwnProperty.call(config, 'key')) {
      var getter = Object.getOwnPropertyDescriptor(config, 'key').get;
      if (getter && getter.isReactWarning) {
        return false;
      }
    }
  }
  return config.key !== undefined;
}
function defineKeyPropWarningGetter(props, displayName) {