Properties for class methods are not set properly

ES6 classes methods should not be enumerable, but code transpiled with buble got these methods enumerable.

Things like
```javascript
class aaa {
bbb() {}
}
```
should be transpiled into:
```javascript
var aaa = function aaa () {};

aaa.prototype.bbb = function bbb () {};
Object.defineProperty(aaa.prototype, 'bbb', {enumerable: false})
```
But the last line doesn't exist. So when trying to traversing the instance of `class aaa` will get `bbb` included, but it shouldn't appear. Hope this could be fixed soon.

Start by locating the class-method transpilation entry point and reproducing the issue with the JavaScript example in the report. Verify that the generated prototype method is non-enumerable, matching the expected Object.defineProperty behavior, and add or update coverage for traversing an instance.

Written by the indexing model from the issue text.