Implement DebugUtils locally

Has been removed from framework

osu.Game/Utils/DebugUtils.cs

@@ -0,0 +1,21 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+namespace osu.Game.Utils
+{
+    public static class DebugUtils
+    {
+        public static bool IsDebug
+        {
+            get
+            {
+                // ReSharper disable once RedundantAssignment
+                bool isDebug = false;
+                // Debug.Assert conditions are only evaluated in debug mode
+                System.Diagnostics.Debug.Assert(isDebug = true);
+                // ReSharper disable once ConditionIsAlwaysTrueOrFalse
+                return isDebug;
+            }
+        }
+    }
+}