Revert "third_party/protobuf: Update to HEAD (83d681ee2c)"

third_party/protobuf/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.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
@@ skipping 16 matching lines @@
 // 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.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;
-#if DOTNET35
-// Needed for ReadOnlyDictionary, which does not exist in .NET 3.5
-using Google.Protobuf.Collections;
-#endif
 
 namespace Google.Protobuf.Reflection
 {
     /// <summary>
     /// Describes a message type.
     /// </summary>
     public sealed class MessageDescriptor : DescriptorBase
     {
         private static readonly HashSet<string> WellKnownTypeNames = new HashSet<string>
         {
@@ skipping 261 matching lines @@
                     if (fieldDescriptor == null)
                     {
                         throw new KeyNotFoundException("No such field name");
                     }
                     return fieldDescriptor;
                 }
             }
         }
     }
 }