**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.

const result = traverseContext;
    const keyUnique = (result[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 (keyUnique && child != null) {
      result[name] = child;
    }
  }
}
/**
 * Flattens children that are typically specified as `props.children`. Any null
 * children will not be included in the resulting object.
 * @return {!object} flattened children keyed by name.
 */
function flattenChildren(
  children: ReactElement<any>,
  selfDebugID?: number,
): ?{ [name: string]: ReactElement<any> } {
  if (children == null) {