Merge pull request #4274 from CyrusNajmabadi/brokenReanalyze

Fix issue where asking the solution crawler to reanalyze projects was a no-op.

src/Features/Core/SolutionCrawler/SolutionCrawlerRegistrationService.cs

@@ -105,7 +105,7 @@ public void Reanalyze(Workspace workspace, IIncrementalAnalyzer analyzer, IEnume
 
                 var solution = workspace.CurrentSolution;
                 var set = new HashSet<DocumentId>(documentIds ?? SpecializedCollections.EmptyEnumerable<DocumentId>());
-                set.Union(projectIds.Select(id => solution.GetProject(id)).SelectMany(p => p.DocumentIds));
+                set.UnionWith(projectIds.Select(id => solution.GetProject(id)).SelectMany(p => p.DocumentIds));
 
                 coordinator.Reanalyze(analyzer, set);
             }