File.Exists() is not null when true (#44310)

* File.Exists() is not null when true

* Fix compile

* Fix compile 2

Author: iSazonov

src/libraries/System.IO.FileSystem/ref/System.IO.FileSystem.cs

@@ -23,7 +23,7 @@ public static void Delete(string path, bool recursive) { }
         public static System.Collections.Generic.IEnumerable<string> EnumerateFileSystemEntries(string path, string searchPattern) { throw null; }
         public static System.Collections.Generic.IEnumerable<string> EnumerateFileSystemEntries(string path, string searchPattern, System.IO.EnumerationOptions enumerationOptions) { throw null; }
         public static System.Collections.Generic.IEnumerable<string> EnumerateFileSystemEntries(string path, string searchPattern, System.IO.SearchOption searchOption) { throw null; }
-        public static bool Exists(string? path) { throw null; }
+        public static bool Exists([System.Diagnostics.CodeAnalysis.NotNullWhen(true)] string? path) { throw null; }
         public static System.DateTime GetCreationTime(string path) { throw null; }
         public static System.DateTime GetCreationTimeUtc(string path) { throw null; }
         public static string GetCurrentDirectory() { throw null; }

src/libraries/System.IO.FileSystem/src/System/IO/File.cs

@@ -5,6 +5,7 @@
 using System.Buffers;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Runtime.ExceptionServices;
 using System.Runtime.Versioning;
@@ -114,7 +115,7 @@ public static void Delete(string path)
         // given by the specified path exists; otherwise, the result is
         // false.  Note that if path describes a directory,
         // Exists will return true.
-        public static bool Exists(string? path)
+        public static bool Exists([NotNullWhen(true)] string? path)
         {
             try
             {

src/libraries/System.Private.CoreLib/src/Internal/IO/File.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Security;
 using System.IO;
 
@@ -18,7 +19,7 @@ internal static partial class File
         // given by the specified path exists; otherwise, the result is
         // false.  Note that if path describes a directory,
         // Exists will return true.
-        public static bool Exists(string? path)
+        public static bool Exists([NotNullWhen(true)] string? path)
         {
             try
             {