Transition to netstandard.

- dotnet5.X => netstandard1.y (where y = x-1).
- DNXCore50 => netstandardapp1.5.
- Applied the same changes to ifdefs.

test/ServerComparison.TestSites/project.json

@@ -17,12 +17,15 @@
     "wwwroot/**/*"
   ],
   "frameworks": {
-    "dnx451": { },
-    "dnxcore50": {
+    "dnx451": {},
+    "netstandardapp1.5": {
       "dependencies": {
         "NETStandard.Library": "1.0.0-*"
       },
-      "imports": "portable-net451+win8"
-     }
+      "imports": [
+        "dnxcore50",
+        "portable-net451+win8"
+      ]
+    }
   }
 }