Add IAsyncDisposable implementations to refs (dotnet/corefx#34898)

Apparently when I added the DisposeAsync methods to the refs, I didn't actually mark the corresponding types as implementing the interface.  This fixes that.

Commit migrated from dotnet/corefx@672d6b0

src/libraries/System.Runtime.Extensions/ref/System.Runtime.Extensions.cs

@@ -1311,7 +1311,7 @@ protected virtual void FillBuffer(int numBytes) { }
         [System.CLSCompliantAttribute(false)]
         public virtual ulong ReadUInt64() { throw null; }
     }
-    public partial class BinaryWriter : System.IDisposable
+    public partial class BinaryWriter : System.IDisposable, System.IAsyncDisposable
     {
         public static readonly System.IO.BinaryWriter Null;
         protected System.IO.Stream OutStream;
@@ -1573,7 +1573,7 @@ protected virtual void Dispose(bool disposing) { }
         public virtual System.Threading.Tasks.Task<string> ReadToEndAsync() { throw null; }
         public static System.IO.TextReader Synchronized(System.IO.TextReader reader) { throw null; }
     }
-    public abstract partial class TextWriter : System.MarshalByRefObject, System.IDisposable
+    public abstract partial class TextWriter : System.MarshalByRefObject, System.IDisposable, System.IAsyncDisposable
     {
         protected char[] CoreNewLine;
         public static readonly System.IO.TextWriter Null;

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -5198,7 +5198,7 @@ public enum SeekOrigin
         Current = 1,
         End = 2,
     }
-    public abstract partial class Stream : System.MarshalByRefObject, System.IDisposable
+    public abstract partial class Stream : System.MarshalByRefObject, System.IDisposable, System.IAsyncDisposable
     {
         public static readonly System.IO.Stream Null;
         protected Stream() { }
@@ -7832,7 +7832,7 @@ public readonly partial struct CancellationToken
         public void ThrowIfCancellationRequested() { }
         public System.Threading.CancellationTokenRegistration UnsafeRegister(System.Action<object> callback, object state) { throw null; }
     }
-    public readonly partial struct CancellationTokenRegistration : System.IDisposable, System.IEquatable<System.Threading.CancellationTokenRegistration>
+    public readonly partial struct CancellationTokenRegistration : System.IDisposable, System.IAsyncDisposable, System.IEquatable<System.Threading.CancellationTokenRegistration>
     {
         private readonly object _dummy;
         private readonly int _dummyPrimitive;

src/libraries/System.Threading.Timer/ref/System.Threading.Timer.cs

@@ -8,7 +8,7 @@
 
 namespace System.Threading
 {
-    public sealed class Timer : System.MarshalByRefObject, System.IDisposable
+    public sealed class Timer : System.MarshalByRefObject, System.IDisposable, System.IAsyncDisposable
     {
         public Timer(System.Threading.TimerCallback callback) { }
         public Timer(System.Threading.TimerCallback callback, object state, int dueTime, int period) { }