Transpiled classes are not block-scoped

[reference](http://exploringjs.com/es6/ch_variables.html). Example:

```js
class C {}
var c1 = C;
{
class C {}
var c2 = C;
}
return C === c1;
```

No source file or test is named. Start by turning the class example in the issue into a reproduction and compare the transpiled behavior with the linked ES6 block-scoping reference; done means the inner class does not replace the outer binding and the final comparison remains correct.

Written by the indexing model from the issue text.