Commented Unassigned: Errors after upgrading from v.3 to v.15 [28039]

After installing the latest version of the toolkit I am getting the errors on every line which uses its methods:

```
AjaxControlToolkit is not declared
```

Example:

```
suggestions.Add(AjaxControlToolkit.AutoCompleteExtender.CreateAutoCompleteItem(GetLoginEmailString(lCurItem), CType(lCurItem("LoginID"), String)))
```

Originally the project used DLL version 3.5.60501 from 2012. I downloaded and installed the latest version, copied the DLL into the References of the project and removed/re-added the reference. Then I followed the doc re upgrading to 15.1 by reverting from ToolkitScriptManager to asp:ScriptManager.

What am I doing wrong?
Comments: Here, I've created a sample project with both 3.5 and 15 toolkit DLLs and flipping between them can reproduce the issue: v.3 no errors on build v.15 error AjaxControlToolkit is not declared.