Let StubProjects correctly return null bin paths

I should have overriden this from the start -- the ExplicitBinPath
didn't give you a way to specify you really did want it to be null.

src/VisualStudio/Core/Def/Implementation/ProjectSystem/AbstractProject.cs

@@ -94,12 +94,17 @@ internal abstract partial class AbstractProject : ForegroundThreadAffinitizedObj
             }
         }
 
-        protected string ExplicitBinPath;
-
         /// <summary>
         /// A full path to the project bin output binary, or null if the project doesn't have an bin output binary.
         /// </summary>
-        internal string BinOutputPath => ExplicitBinPath ?? VisualStudioProject.OutputFilePath;
+        // FYI: this can't be made virtual because there are calls to this where a 'call' instead of 'callvirt' is being used to call
+        // the method.
+        internal string BinOutputPath => GetOutputFilePath();
+
+        protected virtual string GetOutputFilePath()
+        {
+            return VisualStudioProject.OutputFilePath;
+        }
 
         public IReferenceCountedDisposable<IRuleSetFile> RuleSetFile { get; private set; }
 
@@ -128,7 +133,7 @@ internal abstract partial class AbstractProject : ForegroundThreadAffinitizedObj
         public VersionStamp Version { get; }
 
         public IProjectCodeModel ProjectCodeModel { get; protected set; }
-        
+
         /// <summary>
         /// The containing directory of the project. Null if none exists (consider Venus.)
         /// </summary>
@@ -235,7 +240,7 @@ internal IVisualStudioHostDocument GetCurrentDocumentFromPath(string filePath)
             var id = _visualStudioWorkspace.CurrentSolution.GetDocumentIdsWithFilePath(filePath).FirstOrDefault(d => d.ProjectId == Id);
 
             if (id != null)
-            { 
+            {
                 return new DocumentProvider.ShimDocument(this, id, filePath);
             }
             else

src/VisualStudio/Core/Def/Implementation/ProjectSystem/VisualStudioWorkspaceImpl.cs

@@ -202,10 +202,17 @@ internal AbstractProject GetHostProject(ProjectId projectId)
 
         private sealed class StubProject : AbstractProject
         {
+            private readonly string _outputPath;
+
             public StubProject(VisualStudioProjectTracker projectTracker, CodeAnalysis.Project project, IVsHierarchy hierarchy, string outputPath)
                 : base(projectTracker, null, project.Name + "_Stub", null, hierarchy, project.Language, Guid.Empty, null, null, null, null)
             {
-                ExplicitBinPath = outputPath;
+                _outputPath = outputPath;
+            }
+
+            protected override string GetOutputFilePath()
+            {
+                return _outputPath;
             }
         }