Make osu.Game + rulesets compile with netstandard

osu.Game/Utils/ZipUtils.cs

@@ -0,0 +1,33 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu-framework/master/LICENCE
+
+using System;
+using SharpCompress.Archives.Zip;
+
+namespace osu.Game.Utils
+{
+    public static class ZipUtils
+    {
+        public static bool IsZipArchive(string path)
+        {
+            try
+            {
+                using (var arc = ZipArchive.Open(path))
+                {
+                    foreach (var entry in arc.Entries)
+                    {
+                        using (entry.OpenEntryStream())
+                        {
+                        }
+                    }
+                }
+
+                return true;
+            }
+            catch (Exception)
+            {
+                return false;
+            }
+        }
+    }
+}