These are "empty" versions of the libraries that contain only the metadata. They tell your compiler and Visual Studio which classes and methods are available in .NET 4.5.
How to obtain the Developer Pack
Here is the direct download link and information for the . net framework 45 developer pack link
: The reference assemblies required for Visual Studio to list the specific framework version in project settings. IntelliSense Files These are "empty" versions of the libraries that