Observed this on react@16.0.0-beta.5

```js
import React from 'react';
import { renderToString } from 'react-dom/server';

const B = () => 'b';

console.log(renderToString(<div>{'a'}<B />{'c'}</div>));
```

This renders as `<div data-reactroot="">a<!-- -->bc</div>`. If you try to hydrate this you'll get the error: Text content did not match. Server: "bc" Client: "b".
if (this.exhausted) {
      return null;
    }
    var out = '';
    while (out.length < bytes) {
      if (this.stack.length === 0) {
        this.exhausted = true;
        break;
      }
      var frame = this.stack[this.stack.length - 1];
      if (frame.childIndex >= frame.children.length) {
        out += frame.footer;
        this.previousWasTextNode = false;
        this.stack.pop();
        if (frame.tag === 'select') {
          this.currentSelectValue = null;
        }
        continue;
      }
      var child = frame.children[frame.childIndex++];
      if (__DEV__) {