IIFE isolation_mode does not work for chunks

I think `--isolation_mode IIFE` should have the same effect on chunks as on output files. Otherwise chunks pollute the _window_ namespace.

```js
console.log('hello world')
```

```console
java -jar compiler.jar \
--compilation_level ADVANCED \
--isolation_mode IIFE \
--js t.js \
--chunk mychunk:auto \
```

```js
// mychunk.js
console.log("hello world");

```

Although then it's not clear then how one chunk could use data from the dependency chunk... unless via google's module loading irc