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

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

-#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2008 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.Collections;
-
-namespace Google.Protobuf.Reflection
-{
-    /// <summary>
-    /// Allows fields to be reflectively accessed.
-    /// </summary>
-    public interface IFieldAccessor
-    {
-        /// <summary>
-        /// Returns the descriptor associated with this field.
-        /// </summary>
-        FieldDescriptor Descriptor { get; }
-
-        /// <summary>
-        /// Clears the field in the specified message. (For repeated fields,
-        /// this clears the list.)
-        /// </summary>
-        void Clear(IMessage message);
-
-        /// <summary>
-        /// Fetches the field value. For repeated values, this will be an
-        /// <see cref="IList"/> implementation. For map values, this will be an
-        /// <see cref="IDictionary"/> implementation.
-        /// </summary>
-        object GetValue(IMessage message);
-
-        /// <summary>
-        /// Mutator for single "simple" fields only.
-        /// </summary>
-        /// <remarks>
-        /// Repeated fields are mutated by fetching the value and manipulating it as a list.
-        /// Map fields are mutated by fetching the value and manipulating it as a dictionary.
-        /// </remarks>
-        /// <exception cref="InvalidOperationException">The field is not a "simple" field.</exception>
-        void SetValue(IMessage message, object value);
-    }
-}