transformArrowFunction(tree) {
    if (this.inDerivedClass_ && this.inConstructor_) {
      return this.transformUsingCommaExpression_(tree);
    }
    return this.transformUsingTempVar_(tree);
  }
Transforms an arrow function expression into a function declaration.
The main things we need to deal with are the 'this' binding, and adding a
function body and return statement if needed.