parseForOfStatement_(start, initializer) {
    this.eatId_(); // of
    let collection = this.parseExpression_(ALLOW_IN);
    this.eat_(CLOSE_PAREN);
    let body = this.parseStatement_();
    return new ForOfStatement(this.getTreeLocation_(start), initializer,
                              collection, body);
  }
@param {SourcePosition} start
@param {ParseTree} initializer
@return {ParseTree}
@private