Add an empty dependency group for net45 to DataFlow package

DataFlow has assets that are netstandard and old PCL and corrisponding
dependencies. However when selecting for .NET Framework we can eliminate
the dependencies as they aren't necessary as we know they all exist already
in framework.

Fixes https://github.com/dotnet/corefx/issues/25171

Author: weshaggard

src/System.Threading.Tasks.Dataflow/pkg/System.Threading.Tasks.Dataflow.pkgproj

@@ -12,12 +12,20 @@
 
     <!-- Since UAP and .NETCoreApp are package based we still want to enable
          OOBing libraries that happen to overlap with their framework package.
-         This avoids us having to lock the API in our NuGet packages just 
-         to match what shipped inbox: since we can provide a new library 
+         This avoids us having to lock the API in our NuGet packages just
+         to match what shipped inbox: since we can provide a new library
          we can update it to add API without raising the netstandard version. -->
     <ValidatePackageSuppression Include="TreatAsOutOfBox">
       <Value>.NETCoreApp;UAP</Value>
     </ValidatePackageSuppression>
+
+    <!--
+      Include empty dependency group for net45 and higher to avoid unncessary package dependencies
+      because all the dependencies are known to be inbox already.
+    -->
+    <Dependency Include="_._">
+      <TargetFramework>net45</TargetFramework>
+    </Dependency>
   </ItemGroup>
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.targets))\dir.targets" />
 </Project>