Revert https://codereview.chromium.org/1291903002 (protobuf roll).

third_party/protobuf/csharp/src/Google.Protobuf/Reflection/SingleFieldAccessor.cs

-#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2015 Google Inc.  All rights reserved.
-// https://developers.google.com/protocol-buffers/
-//
-// Redistribution and use in source and binary forms, with or without
-// modification, are permitted provided that the following conditions are
-// met:
-//
-//     * Redistributions of source code must retain the above copyright
-// notice, this list of conditions and the following disclaimer.
-//     * Redistributions in binary form must reproduce the above
-// copyright notice, this list of conditions and the following disclaimer
-// in the documentation and/or other materials provided with the
-// distribution.
-//     * Neither the name of Google Inc. nor the names of its
-// contributors may be used to endorse or promote products derived from
-// this software without specific prior written permission.
-//
-// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
-// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
-// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
-// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
-// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
-// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
-// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
-// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
-// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
-// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-#endregion
-
-using System;
-using System.Reflection;
-using Google.Protobuf.Compatibility;
-
-namespace Google.Protobuf.Reflection
-{
-    /// <summary>
-    /// Accessor for single fields.
-    /// </summary>
-    internal sealed class SingleFieldAccessor : FieldAccessorBase
-    {
-        // All the work here is actually done in the constructor - it creates the appropriate delegates.
-        // There are various cases to consider, based on the property type (message, string/bytes, or "genuine" primitive)
-        // and proto2 vs proto3 for non-message types, as proto3 doesn't support "full" presence detection or default
-        // values.
-
-        private readonly Action<IMessage, object> setValueDelegate;
-        private readonly Action<IMessage> clearDelegate;
-
-        internal SingleFieldAccessor(PropertyInfo property, FieldDescriptor descriptor) : base(property, descriptor)
-        {
-            if (!property.CanWrite)
-            {
-                throw new ArgumentException("Not all required properties/methods available");
-            }
-            setValueDelegate = ReflectionUtil.CreateActionIMessageObject(property.GetSetMethod());
-
-            var clrType = property.PropertyType;
-            
-            // TODO: Validate that this is a reasonable single field? (Should be a value type, a message type, or string/ByteString.)
-            object defaultValue =
-                typeof(IMessage).IsAssignableFrom(clrType) ? null
-                : clrType == typeof(string) ? ""
-                : clrType == typeof(ByteString) ? ByteString.Empty
-                : Activator.CreateInstance(clrType);
-            clearDelegate = message => SetValue(message, defaultValue);
-        }
-
-        public override void Clear(IMessage message)
-        {
-            clearDelegate(message);
-        }
-
-        public override void SetValue(IMessage message, object value)
-        {
-            setValueDelegate(message, value);
-        }
-    }
-}