Add default FileSystemWatcher to unityaot profile (case 1344045).

Author: joncham

external/corefx-bugfix/src/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.UnknownUnix.cs

@@ -0,0 +1,31 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+#if MONO && (!MOBILE || UNITY_AOT)
+
+using System;
+using System.IO;
+namespace System.IO.CoreFX
+#else
+
+namespace System.IO
+#endif
+{
+    public partial class FileSystemWatcher
+    {
+        /// <summary>Called when FileSystemWatcher is finalized.</summary>
+        private void FinalizeDispose()
+        {
+        }
+
+        private void StartRaisingEvents()
+        {
+            throw new PlatformNotSupportedException();
+        }
+
+        private void StopRaisingEvents()
+        {
+            throw new PlatformNotSupportedException();
+        }
+    }
+}