Sealing some more sinks.

Rx.NET/Source/src/System.Reactive/Linq/Observable/For.cs

@@ -30,7 +30,7 @@ namespace System.Reactive.Linq.ObservableImpl
                 yield return _resultSelector(item);
         }
 
-        class _ : ConcatSink<TResult>
+        private sealed class _ : ConcatSink<TResult>
         {
             public _(IObserver<TResult> observer, IDisposable cancel)
                 : base(observer, cancel)

Rx.NET/Source/src/System.Reactive/Linq/Observable/If.cs

@@ -19,10 +19,7 @@ namespace System.Reactive.Linq.ObservableImpl
             _elseSource = elseSource;
         }
 
-        public IObservable<TResult> Eval()
-        {
-            return _condition() ? _thenSource : _elseSource;
-        }
+        public IObservable<TResult> Eval() => _condition() ? _thenSource : _elseSource;
 
         protected override IDisposable Run(IObserver<TResult> observer, IDisposable cancel, Action<IDisposable> setSink)
         {
@@ -31,7 +28,7 @@ namespace System.Reactive.Linq.ObservableImpl
             return sink.Run();
         }
 
-        class _ : Sink<TResult>, IObserver<TResult>
+        private sealed class _ : Sink<TResult>, IObserver<TResult>
         {
             private readonly If<TResult> _parent;
 

Rx.NET/Source/src/System.Reactive/Linq/Observable/IgnoreElements.cs

@@ -20,7 +20,7 @@ namespace System.Reactive.Linq.ObservableImpl
             return _source.SubscribeSafe(sink);
         }
 
-        class _ : Sink<TSource>, IObserver<TSource>
+        private sealed class _ : Sink<TSource>, IObserver<TSource>
         {
             public _(IObserver<TSource> observer, IDisposable cancel)
                 : base(observer, cancel)

Rx.NET/Source/src/System.Reactive/Linq/Observable/IsEmpty.cs

@@ -20,7 +20,7 @@ namespace System.Reactive.Linq.ObservableImpl
             return _source.SubscribeSafe(sink);
         }
 
-        class _ : Sink<bool>, IObserver<TSource>
+        private sealed class _ : Sink<bool>, IObserver<TSource>
         {
             public _(IObserver<bool> observer, IDisposable cancel)
                 : base(observer, cancel)

Rx.NET/Source/src/System.Reactive/Linq/Observable/Latest.cs

@@ -18,7 +18,7 @@ namespace System.Reactive.Linq.ObservableImpl
             return new _(subscription);
         }
 
-        class _ : PushToPullSink<TSource, TSource>
+        private sealed class _ : PushToPullSink<TSource, TSource>
         {
             private readonly object _gate;
             private readonly SemaphoreSlim _semaphore;

Rx.NET/Source/src/System.Reactive/Linq/Observable/MostRecent.cs

@@ -19,7 +19,7 @@ namespace System.Reactive.Linq.ObservableImpl
             return new _(_initialValue, subscription);
         }
 
-        class _ : PushToPullSink<TSource, TSource>
+        private sealed class _ : PushToPullSink<TSource, TSource>
         {
             public _(TSource initialValue, IDisposable subscription)
                 : base(subscription)

Rx.NET/Source/src/System.Reactive/Linq/Observable/Next.cs

@@ -18,7 +18,7 @@ namespace System.Reactive.Linq.ObservableImpl
             return new _(subscription);
         }
 
-        class _ : PushToPullSink<TSource, TSource>
+        private sealed class _ : PushToPullSink<TSource, TSource>
         {
             private readonly object _gate;
             private readonly SemaphoreSlim _semaphore;

Rx.NET/Source/src/System.Reactive/Linq/Observable/OfType.cs

@@ -20,7 +20,7 @@ namespace System.Reactive.Linq.ObservableImpl
             return _source.SubscribeSafe(sink);
         }
 
-        class _ : Sink<TResult>, IObserver<TSource>
+        private sealed class _ : Sink<TResult>, IObserver<TSource>
         {
             public _(IObserver<TResult> observer, IDisposable cancel)
                 : base(observer, cancel)

Rx.NET/Source/src/System.Reactive/Linq/Observable/OnErrorResumeNext.cs

@@ -22,7 +22,7 @@ namespace System.Reactive.Linq.ObservableImpl
             return sink.Run(_sources);
         }
 
-        class _ : TailRecursiveSink<TSource>
+        private sealed class _ : TailRecursiveSink<TSource>
         {
             public _(IObserver<TSource> observer, IDisposable cancel)
                 : base(observer, cancel)

Rx.NET/Source/src/System.Reactive/Linq/Observable/While.cs

@@ -30,7 +30,7 @@ namespace System.Reactive.Linq.ObservableImpl
                 yield return _source;
         }
 
-        class _ : ConcatSink<TSource>
+        private sealed class _ : ConcatSink<TSource>
         {
             public _(IObserver<TSource> observer, IDisposable cancel)
                 : base(observer, cancel)