Cannot import goog.modules from an ES6 module

The following code works if `foo` is a `goog.provide` or a `goog.module` with `goog.module.declareLegacyNamespace`, but fails to compile if it is an ordinary `goog.module`:

``` js
import {foo} from 'goog:foo';
```

The error that shows up is

```
es6.js:1: ERROR - required "foo" namespace never provided
import {foo} from 'goog:foo';
```