Write out less data for objects.

src/Compilers/Core/Portable/Serialization/ObjectReader.cs

@@ -32,7 +32,7 @@ internal sealed partial class ObjectReader : IDisposable
         /// this version, just change VersionByte2.
         /// </summary>
         internal const byte VersionByte1 = 0b10101010;
-        internal const byte VersionByte2 = 0b00000111;
+        internal const byte VersionByte2 = 0b00001000;
 
         private readonly BinaryReader _reader;
         private readonly CancellationToken _cancellationToken;
@@ -580,7 +580,6 @@ private object ReadObject()
         {
             var id = _objectReferenceMap.GetNextReferenceId();
 
-            _reader.ReadByte();
             var typeReader = _binderSnapshot.GetTypeReaderFromId(this.ReadInt32());
 
             // recursive: read and construct instance immediately from member elements encoding next in the stream

src/Compilers/Core/Portable/Serialization/ObjectWriter.cs

@@ -751,7 +751,10 @@ private void WriteObjectWorker(IObjectWritable writable)
             _objectReferenceMap.Add(writable);
 
             _writer.Write((byte)EncodingKind.Object);
-            this.WriteKnownType(writable.GetType());
+
+            // Directly write out the type-id for this object.  i.e. no need to write out the 'Type'
+            // tag since we just wrote out the 'Object' tag
+            this.WriteInt32(_binderSnapshot.GetTypeId(writable.GetType()));
             writable.WriteTo(this);
         }