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".
var defaultValue = props.defaultValue;
        
        var textareaChildren = props.children;
        if (textareaChildren != null) {
          if (__DEV__) {
            warning(
              false,
              'Use the `defaultValue` or `value` props instead of setting ' +
                'children on <textarea>.',
            );
          }
          invariant(
            defaultValue == null,
            'If you supply `defaultValue` on a <textarea>, do not pass children.',
          );
          if (Array.isArray(textareaChildren)) {
            invariant(
              textareaChildren.length <= 1,
              '<textarea> can only have at most one child.',
            );
            textareaChildren = textareaChildren[0];
          }
          defaultValue = '' + textareaChildren;
        }
        if (defaultValue == null) {
          defaultValue = '';
        }
        initialValue = defaultValue;
      }
      props = Object.assign({}, props, {