Add star to GeneratorExpression

I use generator in sweet.js.

```js
var a = function* () { yield 1; };

// output
// var a$254 = function () {
//     yield 1;
// };
```

The star disappear from FunctionExpression, but does not disappear from FunctionDeclaration.
I see specification of ES6, but not found this behavior.
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-generator-function-definitions

I want you to add star in the same way as FunctionDeclaration.