TreeWalker functions overridden by unrelated class definition

Ran into this issue, and created a minimal repro example demonstrating the problem:
### To Reproduce:

Run the following code at http://closure-compiler.appspot.com/home

``` js
// ==ClosureCompiler==
// @compilation_level ADVANCED_OPTIMIZATIONS
// @formatting pretty_print
// ==/ClosureCompiler==

var AnyClass = function() {};
AnyClass.prototype.nextNode = function() {};

// Don't strip AnyClass during advanced compilation.
console.log(AnyClass);

var walker = document.createTreeWalker(document);

// Logs undefined instead of walker.nextNode().
console.log(walker.nextNode());
```
### Expected:

The second `console.log()` is passed `walker.nextNode()`.
### Actual:

The second `console.log()` is passed `void 0`.
### Observations:

Explicitly typing the return value (`@type {!TreeWalker}`) does not change the behavior.

Any of the following changes will result in the correct compiler output:
- Removing the nextNode function definition.
- Adding any content inside the nextNode function definition (even dead code like `1;`).

This issue extends to other functions like `previousNode` on TreeWalker as well.

This is also an issue with `document.createDocumentFragment()`:

``` js
// ==ClosureCompiler==
// @compilation_level ADVANCED_OPTIMIZATIONS
// @formatting pretty_print
// ==/ClosureCompiler==

var AnyClass = function() {};
AnyClass.prototype.createDocumentFragment = function() {};

// Don't strip AnyClass during advanced compilation.
console.log(AnyClass);

var fragment = document.createDocumentFragment();

console.log(fragment);
```