Class statics: Arrow functions not supported in goog.defineClass.

The class was defined as followed:

``` javascript
var Test = goog.defineClass(null, {
constructor: function(){},

statics: {
test: () => console.log('This is a test')
}
});
```

http://closure-compiler-debugger.appspot.com/#input0%3Dvar%2520Test%2520%253D%2520goog.defineClass(null%252C%2520%257B%250A%2520%2520constructor%253A%2520function()%257B%257D%252C%250A%250A%2520%2520statics%253A%2520%257B%250A%2520%2520%2520%2520test%253A%2520()%2520%253D%253E%2520console.log('This%2520is%2520a%2520test')%250A%2520%2520%257D%250A%257D)%253B%250A%250ATest.test()%253B%26input1%26conformanceConfig%26externs%26refasterjs-template%26includeDefaultExterns%3D1%26CHECK_TYPES_NEW_INFERENCE%3D1%26CLOSURE_PASS%3D1%26LANG_IN_IS_ES6%3D1%26MISSING_PROPERTIES%3D1%26TRANSPILE%3D1
When compile, it throws a compiling error stating that arrow functions are not supported in goog.defineClass.

Previous version of compiler worked well with static arrow functions.

Please explain or fix it. Thanks!