Using Visual Studio 2012, you open a Visual Studio 2010 project which contains the “Visual Studio 2010 SharePoint Power Tools” Sandboxed Visual WebPart. On building your project you get a compiler error;

The name ‘InitialiseControl’ does not appear in the current context.

During the Visual Studio 2012 project/solution upgrade, one of the Visual WebPart designer files (.ascx.g.cs) was removed from the project (.csproj);

To resolve this situation, open and touch the .ascx file (you could simply add a newline) and save the file;

After saving, the missing designer file is restored, and you should now be able to build your project.