Fix dotnet run double dash passing arguments

When run “dotnet run -- foo”, foo should be the argument passed to the
subject app. After replacing the original parser, dotnet-run did not
utilize the “unparsedtoken” of the parsed result.

To append unparsedtoken to RunCommand’s argument is not straight
forward. RunCommand has an “immutable constructor”, which is a good
thing, so I made update RunCommand’s argument following the immutable
pattern -- create a new object with the original field but only change
the arguments. I also made these filed private set.

test/dotnet.Tests/ParserTests/RunParserTests.cs

@@ -0,0 +1,29 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.Linq;
+using FluentAssertions;
+using Microsoft.DotNet.Tools.Run;
+using Xunit;
+using Xunit.Abstractions;
+using System;
+
+namespace Microsoft.DotNet.Tests.ParserTests
+{
+    public class RunParserTests
+    {
+        public RunParserTests(ITestOutputHelper output)
+        {
+            this.output = output;
+        }
+
+        private readonly ITestOutputHelper output;
+
+        [Fact]
+        public void RunParserCanGetArguementFromDoubleDash()
+        {
+            var runCommand = RunCommand.FromArgs(new[]{ "--", "foo" });
+            runCommand.Args.Single().Should().Be("foo");
+        }
+    }
+}