Revert "Clean up build flavors."

This reverts commit bb813f3d93259705945e88212a980a5f4361c2b4.

Ix.NET/Source/refs/System.Interactive.Providers.Ref/System.Interactive.Providers.Ref.csproj

@@ -3,10 +3,14 @@
   <PropertyGroup>
     <Description>Interactive Extensions Providers Library used to build query providers and express queries over enumerable sequences.</Description>
     <AssemblyTitle>Interactive Extensions - Providers Library</AssemblyTitle>
-    <TargetFrameworks>net461;netstandard2.0</TargetFrameworks>
+    <TargetFrameworks>netstandard1.0;netcoreapp2.0;netstandard2.1</TargetFrameworks>
     <PackageTags>Ix;Interactive;Extensions;Enumerable</PackageTags>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(TargetFramework)' == 'netstandard1.0'">
+    <DefineConstants>$(DefineConstants);CRIPPLED_REFLECTION</DefineConstants>
+  </PropertyGroup>
+
   <ItemGroup>
     <ProjectReference Include="..\System.Interactive.Ref\System.Interactive.Ref.csproj" />
   </ItemGroup>
@@ -14,5 +18,10 @@
   <ItemGroup>
     <Compile Include="..\..\System.Interactive.Providers\**\*.cs" Exclude="..\..\System.Interactive.Providers\obj\**" />
   </ItemGroup>
+  
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard1.0'">
+    <PackageReference Include="System.Linq.Queryable" Version="4.3.0" />
+  </ItemGroup>
+
 
 </Project>

Ix.NET/Source/refs/System.Interactive.Ref/System.Interactive.Ref.csproj

@@ -4,7 +4,7 @@
     <Description>Interactive Extensions Main Library used to express queries over enumerable sequences.</Description>
     <AssemblyTitle>Interactive Extensions - Main Library</AssemblyTitle>
     <Authors>Microsoft</Authors>
-    <TargetFrameworks>net461;netstandard2.0</TargetFrameworks>
+    <TargetFrameworks>netstandard1.0;netcoreapp2.0;netstandard2.1</TargetFrameworks>
     <PackageTags>Ix;Interactive;Extensions;Enumerable</PackageTags>
   </PropertyGroup>