Microsoft has released Windows 10 SDK Preview Build 17666

Things to note:

This build works in conjunction with previously released SDKs and Visual Studio 2017. You can install this SDK and still also continue to submit your apps that target Windows 10 Creators build or earlier to the Store.
The Windows SDK will now formally only be supported by Visual Studio 2017 and greater. You can download the Visual Studio 2017 here.
This build of the Windows SDK will install on Windows 10 Insider Preview and supported Windows operating systems.

  Windows 10 SDK Preview Build 17666 released