**Do you want to request a _feature_ or report a _bug_?**

Report a bug.

**What is the current behavior?**

When testing components, `Warning: ReactComponentTreeDevtool: Missing React element for debugID 1 when building stack` is shown when calling `TestUtils.renderIntoDocument`.

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template: https://jsfiddle.net/reactjs/69z2wepo/).**

https://github.com/angusfretwell/reproduce-debugid-warning (`npm install && npm test`).

**What is the expected behavior?**

According to #7187 this is an internal warning and shouldn't be displayed.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
- react@15.2.1
- react-addons-test-utils@15.2.1
- react-dom@15.2.1

I didn't receive this warning on 15.2.0.

var keyUnique = (childInstances[name] === undefined);
  if (__DEV__) {
    var ReactComponentTreeDevtool = require('ReactComponentTreeDevtool');
    warning(
      keyUnique,
      'flattenChildren(...): Encountered two children with the same key, ' +
      '`%s`. Child keys must be unique; when two children share a key, only ' +
      'the first child will be used.%s',
      KeyEscapeUtils.unescape(name),
      ReactComponentTreeDevtool.getStackAddendumByID(selfDebugID)
    );
  }
  if (child != null && keyUnique) {
    childInstances[name] = instantiateReactComponent(child, true);
  }
}
/**
 * ReactChildReconciler provides helpers for initializing or updating a set of
 * children. Its output is suitable for passing it onto ReactMultiChild which
 * does diffed reordering and insertion.
 */
var ReactChildReconciler = {
  /**
   * Generates a "mount image" for each of the supplied children. In the case
   * of `ReactDOMComponent`, a mount image is a string of markup.
   *