Explicitly import NuGet.targets

NuGet.targets was added as a hard dependency in dotnet#2595, this change will
explicitly import it in Microsoft.Common.CurrentVersion.targets.

cibuild.cmd

@@ -158,7 +158,10 @@ if /i "%TARGET%"=="CoreCLR" (
 :: warning MSB3026: Could not copy "XXX" to "XXX". Beginning retry 1 in 1000ms.
 :: warning MSB3073: Exec task failure (when set to be a warning) -- needed to keep from failing on dev desktops that don't have C++ tools
 :: warning AL1053: The version '1.2.3.4-foo' specified for the 'product version' is not in the normal 'major.minor.build.revision' format
-SET _NOWARN=MSB3277;MSB3026;MSB3073;AL1053
+:: warning MSB4011: Imported target already imported. This will happen when Microsoft.NuGet.ImportAfter.targets installed on the local
+::                  machine still has the import to NuGet.targets which is now in Microsoft.Common.CurrentVersion.targets. This can be
+::                  removed when we want to require VS15.5+ to build locally.
+SET _NOWARN=MSB3277;MSB3026;MSB3073;AL1053;MSB4011
 set MSBUILDBINLOGPATH=%~dp0msbuild_rebuild-%HOST%.binlog
 
 echo.

src/Build/Evaluation/IntrinsicFunctions.cs

@@ -449,6 +449,11 @@ public static string GetMSBuildExtensionsPath()
             return BuildEnvironmentHelper.Instance.MSBuildExtensionsPath;
         }
 
+        public static bool IsRunningFromVisualStudio()
+        {
+            return BuildEnvironmentHelper.Instance.Mode == BuildEnvironmentMode.VisualStudio;
+        }
+
         #region Debug only intrinsics
 
         /// <summary>

src/Tasks/Microsoft.Common.CurrentVersion.targets

@@ -5869,6 +5869,14 @@ Copyright (C) Microsoft Corporation. All rights reserved.
   <!-- This import is temporary and will be removed once it is moved into the silverlight targets -->
   <Import Project="$(MSBuildToolsPath)\Microsoft.Data.Entity.targets" Condition="'$(TargetFrameworkIdentifier)' == 'Silverlight' and Exists('$(MSBuildToolsPath)\Microsoft.Data.Entity.targets')"/>
 
+  <!-- Import NuGet.targets (required for GetReferenceNearestTargetFrameworkTask and used for Restore functionality) -->
+  <PropertyGroup>
+    <NuGetRestoreTargets Condition="'$(NuGetRestoreTargets)'=='' and '$([MSBuild]::IsRunningFromVisualStudio())'=='true'">$(MSBuildToolsPath32)\..\..\..\Common7\IDE\CommonExtensions\Microsoft\NuGet\NuGet.targets</NuGetRestoreTargets>
+    <NuGetRestoreTargets Condition="'$(NuGetRestoreTargets)'==''">$(MSBuildToolsPath)\NuGet.targets</NuGetRestoreTargets>
+  </PropertyGroup>
+
+  <Import Project="$(NuGetRestoreTargets)" />
+
   <Import Project="$(CustomAfterMicrosoftCommonTargets)" Condition="'$(CustomAfterMicrosoftCommonTargets)' != '' and Exists('$(CustomAfterMicrosoftCommonTargets)')"/>
 
   <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.targets\ImportAfter\*" Condition="'$(ImportByWildcardAfterMicrosoftCommonTargets)' == 'true' and exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.targets\ImportAfter')"/>