Switch sharedframework project.json to Microsoft.NETCore.App

Use local-build corehost.exe

This is staying in the repo, so we can copy the local-built version.

Copy deps and deps.json, make sure corehost temp dir exists

Add additional runtimes to the sharedframework's project.json

Delete an unused file, modify crossgen target condition

* sharedhost\project.json is unused. It is copied from the local build.
* the crossgen target was conditioned on an environment variable, but the
  variable could be set to anything. Instead, make sure it is either
  "true" or "1".

Use stage2 instead of stage0 for shared framework restore

scripts/Microsoft.DotNet.Cli.Build.Framework/BuildContext.cs

@@ -22,7 +22,17 @@ namespace Microsoft.DotNet.Cli.Build.Framework
 
         public object this[string name]
         {
-            get { return Properties.ContainsKey(name) ? Properties[name] : null; }
+            get
+            {
+                if (Properties.ContainsKey(name))
+                {
+                    return Properties[name];
+                }
+                else
+                {
+                    throw new KeyNotFoundException("No property with key " + name + " was found.");
+                }
+            }
             set { Properties[name] = value; }
         }