Transfer import port yahoo emails to gmail google email account

Step 1: Login to gmail
Step 2:
Step 3:
Step 4:
Following instructions
Finally, after a few hours:
after import, your original emails are still intact.
References:
Google troubleshooter: https://support.google.com/mail/answer/164640?hl=en&rd=1
Google gmail forum: https://productforums.google.com/forum/#!forum/gmail

A problem I encountered: for one yahoo account gmail said it's completed, but I can not find any imported emails.
that yahoo email account is for disposable, for junk email, there are about 70K pieces of emails, many has big attachment,
I cleaned up half of them, restarted the import in Gmail, then it's OK.