Remove xUnit dependency by internalizing assert source

NuGet.Config

@@ -2,6 +2,7 @@
 <configuration>
   <packageSources>
     <add key="Build Packages" value="https://www.myget.org/F/c037199d-41df-4567-b966-25ff65324688/api/v3/index.json" />
+    <add key="CI Builds (xunit)" value="https://www.myget.org/F/xunit/" />
     <add key="NuGet.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
 </configuration>

Rx.NET/Source/Microsoft.Reactive.Testing/project.json

@@ -15,7 +15,7 @@
 
   "buildOptions": {
     "keyFile": "../ReactiveX.snk",
-    "define": [ "SIGNED" ],
+    "define": [ "SIGNED", "PLATFORM_DOTNET", "XUNIT_VISIBILITY_INTERNAL" ],
     "nowarn": [ "CS1591" ],
     "xmlDoc": true,
     "compile": {
@@ -26,7 +26,7 @@
   "dependencies": {
     "System.Reactive.Linq": { "target": "project" },
     "System.Reactive.PlatformServices": { "target": "project" },
-    "xunit.assert": "2.2.0-beta2-build3300"
+    "xunit.assert.source": "2.2.0-beta3-build3316"
   },
   
   "frameworks": {
@@ -77,7 +77,6 @@
       }
     },
     "netstandard1.0": {
-      "imports": ["dotnet", "portable-net45+win8"],
       "buildOptions": {
         "define": [
           "NO_EVENTARGS_CONSTRAINT",
@@ -101,7 +100,6 @@
       }
     },
     "netstandard1.3": {
-      "imports": ["dotnet", "portable-net45+win8"],
       "buildOptions": {
         "define": [
           "NO_EVENTARGS_CONSTRAINT",

Rx.NET/Source/NuSpecs/Microsoft.Reactive.Testing.nuspec

@@ -16,7 +16,6 @@
     <tags>Rx Reactive Extensions Observable LINQ Events</tags>
     <dependencies>
       <dependency id="System.Reactive.PlatformServices" version="$version$" />
-      <dependency id="xunit.assert" version="2.1.0" />
     </dependencies>
   </metadata>
   <files>