Before most of the code in the designer was using EF6 but validation still used EF5. In EF6 we introduced new manifest tokens for Sql Server (one for 2012 and one for Azure) and the code that was still using EF5 would fail on new manifest tokens. To workaround this for now we added the PreviewHelper class that was rewriting newly added manifest tokens to 2008. Once we move validation to EF6 the class must be removed and we should use the new manifest token for EF6 models.
Note that for EF5 models we must not use new manifest tokens.
Note that for EF5 models we must not use new manifest tokens.