Enable win32 file dialog functionality for .NET Standard build

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -841,14 +841,13 @@ namespace Avalonia.Win32.Interop
 
             return SetClassLong64(hWnd, nIndex, dwNewLong);
         }
-#if !NETSTANDARD && !NETSTANDARD2_0
+
         [ComImport, ClassInterface(ClassInterfaceType.None), TypeLibType(TypeLibTypeFlags.FCanCreate), Guid("DC1C5A9C-E88A-4DDE-A5A1-60F82A20AEF7")]
         internal class FileOpenDialogRCW { }
 
         
         [DllImport("shell32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
         internal static extern int SHCreateItemFromParsingName([MarshalAs(UnmanagedType.LPWStr)] string pszPath, IntPtr pbc, ref Guid riid, [MarshalAs(UnmanagedType.Interface)] out IShellItem ppv);
-#endif
 
         [DllImport("user32.dll", SetLastError = true)]
         public static extern bool OpenClipboard(IntPtr hWndOwner);
@@ -1183,7 +1182,7 @@ namespace Avalonia.Win32.Interop
             public int flagsEx;
         }        
     }
-#if !NETSTANDARD && !NETSTANDARD1_3
+
     [ComImport(), Guid("42F85136-DB7E-439C-85F1-E4075D135FC8"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
     internal interface IFileDialog
     {
@@ -1283,5 +1282,4 @@ namespace Avalonia.Win32.Interop
         uint Compare([In, MarshalAs(UnmanagedType.Interface)] IShellItem psi, [In] uint hint, out int piOrder);
         
     }
-#endif
 }

src/Windows/Avalonia.Win32/SystemDialogImpl.cs

@@ -100,17 +100,14 @@ namespace Avalonia.Win32
                     var pofn = &ofn;
 
                     // We should save the current directory to restore it later.
-#if !NETSTANDARD
                     var currentDirectory = Environment.CurrentDirectory;
-#endif
+
                     var res = dialog is OpenFileDialog
                         ? UnmanagedMethods.GetOpenFileName(new IntPtr(pofn))
                         : UnmanagedMethods.GetSaveFileName(new IntPtr(pofn));
 
                     // Restore the old current directory, since GetOpenFileName and GetSaveFileName change it after they're called
-#if !NETSTANDARD
                     Environment.CurrentDirectory = currentDirectory;
-#endif
 
                     if (!res)
                         return null;
@@ -155,9 +152,6 @@ namespace Avalonia.Win32
 
         public Task<string> ShowFolderDialogAsync(OpenFolderDialog dialog, IWindowImpl parent)
         {
-#if NETSTANDARD
-            throw new NotImplementedException();
-#else
             return Task.Factory.StartNew(() =>
             {
                 string result = string.Empty;
@@ -214,7 +208,6 @@ namespace Avalonia.Win32
 
                 return result;
             });
-#endif
         }
     }
 }