Change HostProjectBuildComplete to persist workspace projects.

- This was missed in the VS15.7 movement to dev.

src/Microsoft.CodeAnalysis.Razor.Workspaces/ProjectSystem/DefaultProjectSnapshotManager.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 
 namespace Microsoft.CodeAnalysis.Razor.ProjectSystem
@@ -218,9 +219,18 @@ namespace Microsoft.CodeAnalysis.Razor.ProjectSystem
 
             if (_projects.TryGetValue(hostProject.FilePath, out var original))
             {
+                var workspaceProject = GetWorkspaceProject(hostProject.FilePath);
+                if (workspaceProject == null)
+                {
+                    // Host project was built prior to a workspace project being associated. We have nothing to do without
+                    // a workspace project so we short circuit.
+                    return;
+                }
+
                 // Doing an update to the project should keep computed values, but mark the project as dirty if the
                 // underlying project is newer.
-                var snapshot = original.WithHostProject(hostProject);
+                var snapshot = original.WithWorkspaceProject(workspaceProject);
+
                 _projects[hostProject.FilePath] = snapshot;
 
                 // Notify the background worker so it can trigger tag helper discovery.

test/Microsoft.VisualStudio.LanguageServices.Razor.Test/ProjectSystem/DefaultProjectSnapshotManagerTest.cs

@@ -72,6 +72,28 @@ namespace Microsoft.CodeAnalysis.Razor.ProjectSystem
 
         private Workspace Workspace { get; }
 
+        [ForegroundFact]
+        public void HostProjectBuildComplete_FindsChangedWorkspaceProject_AndStartsBackgroundWorker()
+        {
+            // Arrange
+            Assert.True(Workspace.TryApplyChanges(WorkspaceProject.Solution));
+            ProjectManager.HostProjectAdded(HostProject);
+            var project = WorkspaceProject.WithAssemblyName("Test1"); // Simulate a project change
+            ProjectManager.WorkspaceProjectAdded(project);
+            ProjectManager.Reset();
+
+            // Act
+            ProjectManager.HostProjectBuildComplete(HostProject);
+
+            // Assert
+            var snapshot = ProjectManager.GetSnapshot(HostProject);
+            Assert.True(snapshot.IsDirty);
+            Assert.True(snapshot.IsInitialized);
+
+            Assert.False(ProjectManager.ListenersNotified);
+            Assert.True(ProjectManager.WorkerStarted);
+        }
+
         [ForegroundFact]
         public void HostProjectAdded_WithoutWorkspaceProject_NotifiesListeners()
         {