Add ContractAnnotation to Contract.Requires.

To tell resharper that an exception will be thrown when condition ==
false.

src/Perspex.Base/Contract.cs

@@ -1,12 +1,12 @@
 // Copyright (c) The Perspex Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System;
 using System.Runtime.CompilerServices;
+using JetBrains.Annotations;
 
 namespace Perspex
 {
-    using System;
-
     /// <summary>
     /// A stub of Code Contract's Contract class.
     /// </summary>
@@ -25,6 +25,7 @@ namespace Perspex
         /// </typeparam>
         /// <param name="condition">The precondition.</param>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        [ContractAnnotation("condition:false=>stop")]
         public static void Requires<TException>(bool condition) where TException : Exception, new()
         {
             if (!condition)

src/Perspex.Base/packages.config

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
+  <package id="JetBrains.Annotations" version="9.2.0" targetFramework="portable45-net45+win8" />
   <package id="Rx-Core" version="2.2.5" targetFramework="portable-net45+win8" />
   <package id="Rx-Interfaces" version="2.2.5" targetFramework="portable-net45+win8" />
   <package id="Rx-Linq" version="2.2.5" targetFramework="portable-net45+win8" />