Revert "Update basic middleware projects to netcoreapp3.0"

This partially reverts commit e0bbda07fc.

src/Middleware/HostFiltering/src/Microsoft.AspNetCore.HostFiltering.csproj

@@ -4,7 +4,7 @@
     <Description>
       ASP.NET Core middleware for filtering out requests with unknown HTTP host headers.
     </Description>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageTags>aspnetcore</PackageTags>
   </PropertyGroup>
@@ -13,6 +13,6 @@
     <Reference Include="Microsoft.AspNetCore.Http" />
     <Reference Include="Microsoft.AspNetCore.Http.Extensions" />
     <Reference Include="Microsoft.Extensions.Options" />
-    <Reference Include="Microsoft.AspNetCore.Hosting.Abstractions" />
+    <Reference Include="Microsoft.AspNetCore.Hosting.Abstractions" /> 
   </ItemGroup>
 </Project>

src/Middleware/HttpsPolicy/src/Microsoft.AspNetCore.HttpsPolicy.csproj

@@ -4,7 +4,7 @@
     <Description>
       ASP.NET Core basic middleware for supporting HTTPS Redirection and HTTP Strict-Transport-Security.
     </Description>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
     <NoWarn>$(NoWarn);CS1591</NoWarn>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageTags>aspnetcore;https;hsts</PackageTags>
@@ -15,6 +15,6 @@
     <Reference Include="Microsoft.AspNetCore.Http.Extensions" />
     <Reference Include="Microsoft.Extensions.Configuration.Binder" />
     <Reference Include="Microsoft.Extensions.Options" />
-    <Reference Include="Microsoft.AspNetCore.Hosting.Abstractions" />
+    <Reference Include="Microsoft.AspNetCore.Hosting.Abstractions" /> 
   </ItemGroup>
 </Project>

src/Middleware/ResponseCompression/src/BrotliCompressionProvider.cs

@@ -37,6 +37,15 @@ namespace Microsoft.AspNetCore.ResponseCompression
 
         /// <inheritdoc />
         public Stream CreateStream(Stream outputStream)
-            => new BrotliStream(outputStream, Options.Level, leaveOpen: true);
+        {
+#if NETCOREAPP2_1
+            return new BrotliStream(outputStream, Options.Level, leaveOpen: true);
+#elif NET461 || NETSTANDARD2_0
+            // Brotli is only supported in .NET Core 2.1+
+            throw new PlatformNotSupportedException();
+#else
+#error Target frameworks need to be updated.
+#endif
+        }
     }
 }

src/Middleware/ResponseCompression/src/GzipCompressionProvider.cs

@@ -33,7 +33,19 @@ namespace Microsoft.AspNetCore.ResponseCompression
         public string EncodingName { get; } = "gzip";
 
         /// <inheritdoc />
-        public bool SupportsFlush { get; } = true;
+        public bool SupportsFlush
+        {
+            get
+            {
+#if NET461
+                return false;
+#elif NETSTANDARD2_0 || NETCOREAPP2_1
+                return true;
+#else
+#error target frameworks need to be updated
+#endif
+            }
+        }
 
         /// <inheritdoc />
         public Stream CreateStream(Stream outputStream)

src/Middleware/ResponseCompression/src/Microsoft.AspNetCore.ResponseCompression.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <Description>ASP.NET Core middleware for HTTP Response compression.</Description>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFrameworks>net461;netstandard2.0;netcoreapp2.1</TargetFrameworks>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageTags>aspnetcore</PackageTags>
   </PropertyGroup>

src/Middleware/ResponseCompression/src/ResponseCompressionProvider.cs

@@ -48,7 +48,13 @@ namespace Microsoft.AspNetCore.ResponseCompression
                 // Use the factory so it can resolve IOptions<GzipCompressionProviderOptions> from DI.
                 _providers = new ICompressionProvider[]
                 {
+#if NETCOREAPP2_1
                     new CompressionProviderFactory(typeof(BrotliCompressionProvider)),
+#elif NET461 || NETSTANDARD2_0
+                    // Brotli is only supported in .NET Core 2.1+
+#else
+#error Target frameworks need to be updated.
+#endif
                     new CompressionProviderFactory(typeof(GzipCompressionProvider)),
                 };
             }

src/Middleware/Rewrite/src/Microsoft.AspNetCore.Rewrite.csproj

@@ -5,7 +5,7 @@
 * Support for custom URL rewrite rules
 * Support for running IIS URL Rewrite module rules
 * Support for running Apache mod_rewrite rules.</Description>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
     <NoWarn>$(NoWarn);CS1591</NoWarn>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageTags>aspnetcore;urlrewrite;mod_rewrite</PackageTags>