parseGeneratorComprehension_(start) {
    let comprehensionList = this.parseComprehensionList_();
    let expression = this.parseAssignmentExpression_(ALLOW_IN);
    this.eat_(CLOSE_PAREN);
    return new GeneratorComprehension(this.getTreeLocation_(start),
                                      comprehensionList,
                                      expression);
  }
Continues parsing generator expressions. The opening paren and the
expression is parsed by parseArrowFunction_.

https://bugs.ecmascript.org/show_bug.cgi?id=381

GeneratorComprehension :
  ( Comprehension )