The quotation DOES appear in the released complete email chain. See the email on June 3, 2016 at 10:36 AM from Rob Goldstone to Trump, Jr., as tweeted by Trump himself:

"The Crown prosecutor of Russia…offered to provide the Trump campaign with some official documents and information that would incriminate Hillary and her dealings with Russia and would be very useful to your father.
This is obviously very high level and sensitive information but is part of Russia and its government’s support for Mr Trump – helped along by Aras and Emin. What do you think is the best way to handle this information and would you be able to speak to Emin about it directly? I can also send this info to your father"