Removed HttpClientHandler dead code and folded .Core file.

src/libraries/System.Net.Http/src/System.Net.Http.csproj

@@ -31,7 +31,6 @@
     <Compile Include="System\Net\Http\HttpBaseStream.cs" />
     <Compile Include="System\Net\Http\HttpClient.cs" />
     <Compile Include="System\Net\Http\HttpClientHandler.cs" />
-    <Compile Include="System\Net\Http\HttpClientHandler.Core.cs" />
     <Compile Include="System\Net\Http\HttpCompletionOption.cs" />
     <Compile Include="System\Net\Http\HttpContent.cs" />
     <Compile Include="System\Net\Http\HttpMessageHandler.cs" />

src/libraries/System.Net.Http/src/System/Net/Http/HttpClientHandler.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Globalization;
 using System.Net.Security;
 using System.Collections.Generic;
 using System.Security.Authentication;
@@ -21,6 +22,8 @@ public partial class HttpClientHandler : HttpMessageHandler
         private readonly DiagnosticsHandler _diagnosticsHandler;
         private ClientCertificateOption _clientCertificateOptions;
 
+        private volatile bool _disposed = false;
+
         public HttpClientHandler()
         {
 #if TARGETS_BROWSER
@@ -143,6 +146,39 @@ public int MaxConnectionsPerServer
             set => _underlyingHandler.MaxConnectionsPerServer = value;
         }
 
+        public long MaxRequestContentBufferSize
+        {
+            // This property is not supported. In the .NET Framework it was only used when the handler needed to
+            // automatically buffer the request content. That only happened if neither 'Content-Length' nor
+            // 'Transfer-Encoding: chunked' request headers were specified. So, the handler thus needed to buffer
+            // in the request content to determine its length and then would choose 'Content-Length' semantics when
+            // POST'ing. In .NET Core, the handler will resolve the ambiguity by always choosing
+            // 'Transfer-Encoding: chunked'. The handler will never automatically buffer in the request content.
+            get
+            {
+                return 0; // Returning zero is appropriate since in .NET Framework it means no limit.
+            }
+
+            set
+            {
+                if (value < 0)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(value));
+                }
+
+                if (value > HttpContent.MaxBufferSize)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(value), value,
+                        SR.Format(CultureInfo.InvariantCulture, SR.net_http_content_buffersize_limit,
+                        HttpContent.MaxBufferSize));
+                }
+
+                CheckDisposed();
+
+                // No-op on property setter.
+            }
+        }
+
         public int MaxResponseHeadersLength
         {
             get => _underlyingHandler.MaxResponseHeadersLength;
@@ -264,5 +300,13 @@ private void ThrowForModifiedManagedSslOptionsIfStarted()
             // SslOptions is changed, since SslOptions itself does not do any such checks.
             _underlyingHandler.SslOptions = _underlyingHandler.SslOptions;
         }
+
+        private void CheckDisposed()
+        {
+            if (_disposed)
+            {
+                throw new ObjectDisposedException(GetType().ToString());
+            }
+        }
     }
 }