**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
itRenders(
        'a newline-eating tag with content not starting with \\n',
        async render => {
          const e = await render(<pre>Hello</pre>);
          expect(e.textContent).toBe('Hello');
        },
      );
      itRenders(
        'a newline-eating tag with content starting with \\n',
        async render => {
          const e = await render(<pre>{'\nHello'}</pre>);
          expect(e.textContent).toBe('\nHello');
        },
      );
      itRenders('a normal tag with content starting with \\n', async render => {
        const e = await render(<div>{'\nHello'}</div>);
        expect(e.textContent).toBe('\nHello');
      });
    });
    describe('different component implementations', function() {
      function checkFooDiv(e) {
        expect(e.childNodes.length).toBe(1);
        expectNode(e.firstChild, TEXT_NODE_TYPE, 'foo');
      }
      itRenders('stateless components', async render => {
        const StatelessComponent = () => <div>foo</div>;
        checkFooDiv(await render(<StatelessComponent />));
      });
      itRenders('ES6 class components', async render => {
        class ClassComponent extends React.Component {