**Do you want to request a *feature* or report a *bug*?**

bug

**What is the current behavior?**

When hydrating a pre-rendered component that uses `dangerouslySetInnerHTML`, + an object with a `toString` method, we get a warning such as:

```
Warning: Did not expect server HTML to contain the text node "Bonjour" in <p>.
```

The component will also appear blank.

This is noteworthy since:
- The markup returned by `render` appears to be correct, meaning:
  - The markup that an SSR server returns is correct
  - The markup that appears on page if we just use `render` on the client, without SSR is correct
- It's only if we do both, that we get errors.

**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 for React 16: https://jsfiddle.net/Luktwrdm/, template for React 15: https://jsfiddle.net/hmbg7e9w/).**

https://codesandbox.io/s/vqkq34o965

(Code, for posterity:)
```js
import React from "react";
import { hydrate } from "react-dom";

const HelloObj = {
  toString: () => "Bonjour"
};

const MyComponent = () => (
  <p
    dangerouslySetInnerHTML={{
      __html: HelloObj
    }}
  />
);

hydrate(<MyComponent />, document.getElementById("app"));
```

**What is the expected behavior?**

The component is not blank, and no error is outputted in the console

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

- react/react-dom 16.2.0
  - This worked < 16 (confirmed working in 15.6.1, if we use the `render` method)
- Chrome Version 62.0.3202.94
- OS X 10.12.6
propKey,
            nextProp,
          );
        }
        if (nextProp !== serverValue && !isMismatchDueToBadCasing) {
          warnForPropDifference(propKey, serverValue, nextProp);
        }
      }
    }
  }
  if (__DEV__) {
    
    if (extraAttributeNames.size > 0 && !suppressHydrationWarning) {
      
      warnForExtraAttributes(extraAttributeNames);
    }
  }
  switch (tag) {
    case 'input':
      
      
      inputValueTracking.track((domElement: any));
      ReactDOMFiberInput.postMountWrapper(domElement, rawProps, true);
      break;
    case 'textarea':
      
      
      inputValueTracking.track((domElement: any));
      ReactDOMFiberTextarea.postMountWrapper(domElement, rawProps);
      break;