Quick question - is what Microsoft now calls the \"Microsoft .NET Framework 4.5.2 Developer Pack\" what was historically called the SDK?
The developer pack doesn't have anything to do with an SDK. It just contains the reference assemblies that you need to compile a project that explicitly targets 4.5.2. Without it you couldn't select 4.5.2 from the Project + Properties, Application tab, Target framework version combobox.
The reference assemblies in the C:\Program Files (x86)\Reference Assemblies directory normally are put there by the VS installer. But 4.5.2 shipped too late to have them available, nor is it covered by the Updates.
Do keep in mind that it is very rarely useful to target 4.5.2. It has exceedingly few new additions to the framework, it is mostly a maintenance release. Not unlike 4.0.1, 4.0.2 and 4.0.3, also widely ignored. If you do target it then you're likely to force your user to update their .NET install, it isn't being distributed by Windows Update as yet.
Just a piece of history for reference.
When .NET Framework was 1.0 and 1.1, you need to have the .NET Framework SDK installed, so as to perform most command line tasks. That was how things got started.
.NET Framework 2.0 followed that tradition, has an SDK of its own. Later, Microsoft decided to merge .NET Framework SDK into Windows SDK.
You can find out such information by reading SharpDevelop's documentation on what are the dependencies. As a third party IDE, it always tells which are needed to build a project by assembling Microsoft's installers.
Recently .NET Framework has been updated more frequently, so Microsoft decided to ship Developer Pack as separate downloads. Like Hans Passant explained, this design is more closely related to Visual Studio compilation (referenced assemblies). It is more like a reorganization of different tools and utilities,