Update Settings.targets to import the legacy VSSDK preview bits if we are not on a willow install.

Restore.cmd

@@ -41,6 +41,9 @@ call "%NugetExe%" restore "%RoslynRoot%build\ToolsetPackages\dev14.project.json"
 echo Restoring packages: Toolsets (Dev15 VS SDK build tools)
 call "%NugetExe%" restore "%RoslynRoot%build\ToolsetPackages\dev15.project.json" %NuGetAdditionalCommandLineArgs% || goto :RestoreFailed
 
+echo Restoring packages: Toolsets (Dev15 VS SDK 'Willow' build tools)
+call "%NugetExe%" restore "%RoslynRoot%build\ToolsetPackages\dev15Willow.project.json" %NuGetAdditionalCommandLineArgs% || goto :RestoreFailed
+
 echo Restoring packages: Roslyn SDK
 call "%NugetExe%" restore "%RoslynRoot%build\ToolsetPackages\roslynsdk.project.json" %NuGetAdditionalCommandLineArgs% || goto :RestoreFailed
 

build/Targets/Settings.targets

@@ -128,9 +128,23 @@
       </PropertyGroup>
     </Otherwise>
   </Choose>
+  
+  <!-- This is a really hacky way to detect whether we are on a legacy or a willow based VS install.
+       Basically, we check for a registry key that will only exist in legacy VS installs, and assume
+       we are a willow based installation if our VSVersion is 15.0 and the registry key doesn't exist. -->
+  <PropertyGroup>
+    <VisualStudioInstallDirectoryFromRegistry Condition="'$(OS)' == 'Windows_NT'">$(registry:HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\15.0@InstallDir)</VisualStudioInstallDirectoryFromRegistry>
+    <VisualStudioInstallDirectoryFromRegistry Condition="'$(OS)' == 'Windows_NT' AND '$(VisualStudioInstallDirectoryFromRegistry)' == ''">$(registry:HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\VisualStudio\15.0@InstallDir)</VisualStudioInstallDirectoryFromRegistry>
+  </PropertyGroup>
 
   <Choose>
-    <When Condition="'$(VisualStudioVersion)' == '15.0'">
+    <When Condition="'$(VisualStudioVersion)' == '15.0' AND '$(VisualStudioInstallDirectoryFromRegistry)' != ''">
+      <PropertyGroup>
+        <VisualStudioBuildToolsNuGetPackagePath>$(NuGetPackageRoot)\Microsoft.VSSDK.BuildTools\15.0.25201-dev15preview2</VisualStudioBuildToolsNuGetPackagePath>
+      </PropertyGroup>
+    </When>
+    
+    <When Condition="'$(VisualStudioVersion)' == '15.0' AND '$(VisualStudioInstallDirectoryFromRegistry)' == ''">
       <PropertyGroup>
         <VisualStudioBuildToolsNuGetPackagePath>$(NuGetPackageRoot)\Microsoft.VSSDK.BuildTools\15.0.25604-Preview4</VisualStudioBuildToolsNuGetPackagePath>
       </PropertyGroup>

build/ToolsetPackages/dev15.project.json

 {
     "dependencies": {
-        "Microsoft.VSSDK.BuildTools": "15.0.25604-Preview4"
+        "Microsoft.VSSDK.BuildTools": "15.0.25201-dev15preview2"
     },
     "frameworks": {
         "net46": {}

build/ToolsetPackages/dev15Willow.project.json

+{
+    "dependencies": {
+        "Microsoft.VSSDK.BuildTools": "15.0.25604-Preview4"
+    },
+    "frameworks": {
+        "net46": {}
+    }
+}
+