Improve null asserts (#249)

***NO_CI***

Author: CoryCharlton

source/TestFramework/Assert.cs

@@ -6,6 +6,7 @@
 
 using System;
 using System.Collections;
+using System.Diagnostics.CodeAnalysis;
 using TestFrameworkShared;
 
 namespace nanoFramework.TestFramework
@@ -1604,7 +1605,7 @@ public static void NotSame(
         /// <exception cref="AssertFailedException">Thrown if value is not null.</exception>
         public static void IsNull(object value, string message = "")
         {
-            if (value != null)
+            if (value is not null)
             {
                 HandleFail("Assert.IsNull", message);
             }
@@ -1624,10 +1625,10 @@ public static void IsNull(object value, string message = "")
         /// </summary>
         /// <param name="value">The object the test expects not to be null.</param>
         /// <param name="message">The message to include in the exception when value is null. The message is shown in test results.</param>
-        /// <exception cref="AssertFailedException">Thrown if value is null./exception>
-        public static void IsNotNull(object value, string message = "")
+        /// <exception cref="AssertFailedException">Thrown if value is null.</exception>
+        public static void IsNotNull([NotNull] object value, string message = "")
         {
-            if (value == null)
+            if (value is null)
             {
                 HandleFail("Assert.IsNotNull", message);
             }

source/TestFramework/nanoFramework.TestFramework.nfproj

@@ -47,6 +47,12 @@
       <HintPath>..\..\packages\nanoFramework.CoreLibrary.1.15.5\lib\mscorlib.dll</HintPath>
       <Private>True</Private>
     </Reference>
+    <Reference Include="nanoFramework.System.Runtime">
+      <HintPath>..\..\packages\nanoFramework.System.Runtime.1.0.6\lib\nanoFramework.System.Runtime.dll</HintPath>
+    </Reference>
+  </ItemGroup>
+  <ItemGroup>
+    <Content Include="packages.lock.json" />
   </ItemGroup>
   <Import Project="..\TestFrameworkShared\TestFrameworkShared.projitems" Label="Shared" />
   <Import Project="$(NanoFrameworkProjectSystemPath)NFProjectSystem.CSharp.targets" Condition="Exists('$(NanoFrameworkProjectSystemPath)NFProjectSystem.CSharp.targets')" />

source/TestFramework/packages.config

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
   <package id="nanoFramework.CoreLibrary" version="1.15.5" targetFramework="netnano1.0" />
+  <package id="nanoFramework.System.Runtime" version="1.0.6" targetFramework="netnano1.0" />
   <package id="Nerdbank.GitVersioning" version="3.6.133" developmentDependency="true" targetFramework="netnano1.0" />
 </packages>

source/TestFramework/packages.lock.json

@@ -8,6 +8,12 @@
         "resolved": "1.15.5",
         "contentHash": "u2+GvAp1uxLrGdILACAZy+EVKOs28EQ52j8Lz7599egXZ3GBGejjnR2ofhjMQwzrJLlgtyrsx8nSLngDfJNsAg=="
       },
+      "nanoFramework.System.Runtime": {
+        "type": "Direct",
+        "requested": "[1.0.6, 1.0.6]",
+        "resolved": "1.0.6",
+        "contentHash": "n87itPUMSsOJkUsdoXr0vhiBTggZBMgCtIIC7c+RsVAhF2u/0TU/h+ZLNyFL8Xhl0taPcTN4LiPPTkI+e95Q/g=="
+      },
       "Nerdbank.GitVersioning": {
         "type": "Direct",
         "requested": "[3.6.133, 3.6.133]",