Piotrp msft/msbuild/incremental test (#3842)

* Incremental Testing - Part 1

* Fix 'clean' bug

* Remove --fallbacksource from TestAssetPackage restore.

build_projects/dotnet-cli-build/DotNetRestore.cs

@@ -0,0 +1,42 @@
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+
+namespace Microsoft.DotNet.Cli.Build
+{
+    public class DotNetRestore : DotNetTool
+    {
+        protected override string Command
+        {
+            get { return "restore"; }
+        }
+
+        protected override string Args
+        {
+            get { return $"{GetVerbosity()} {GetFallbackSource()}"; }
+        }
+
+        public string FallbackSource { get; set; }
+
+        public string Verbosity { get; set; }
+
+        private string GetFallbackSource()
+        {
+            if (!string.IsNullOrEmpty(FallbackSource))
+            {
+                return $"--fallbacksource {FallbackSource}";
+            }
+
+            return null;
+        }
+
+        private string GetVerbosity()
+        {
+            if (!string.IsNullOrEmpty(Verbosity))
+            {
+                return $"--verbosity {Verbosity}";
+            }
+
+            return null;
+        }
+    }
+}