Make our simple Contract class throw an exception.

Perspex/Contract.cs

@@ -8,8 +8,14 @@ namespace Perspex
 {
     internal static class Contract
     {
-        public static void Requires<TException>(bool condition) where TException : Exception
+        public static void Requires<TException>(bool condition) where TException : Exception, new()
         {
+#if DEBUG
+            if (!condition)
+            {
+                throw new TException();
+            }
+#endif
         }
     }
 }