I just went through this exercise when upgrading to EF 6 from EF 5 and I had the same errors.
Here is what I had to do.
Install-Package Microsoft.AspNet.EntityDataSource
It would register a new EntityDataSource Control in the web.config under pages:
<pages>
<controls>
<add tagPrefix="ef" assembly="Microsoft.AspNet.EntityDataSource" namespace="Microsoft.AspNet.EntityDataSource" />
</controls>
</pages>
Next step is to replace existing <asp:EntityDataSource />
controls to <ef:EntityDataSource />
in your aspx pages.
Final step is to go in your code behind and update references for EntityDataSourceContextCreatingEventArgs or any other type of EFContext tags.
From
protected void OnContextCreating(object sender, EntityDataSourceContextCreatingEventArgs e){... }
To
protected void OnContextCreating(object sender, Microsoft.AspNet.EntityDataSource.EntityDataSourceContextCreatingEventArgs e){... }
It all worked and I didn't have to reference System.Data.Entity in the assembly.