Some more perf optimizations here

```
Source MonoDevelop.Core.AsyncEvents
00:02:34.7219796 - MonoDevelop.Projects.Project Void OnFileCreated(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:01.5497746 - MonoDevelop.Projects.Project Void OnFileDeleted(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.0229513 - MonoDevelop.Projects.Project Void OnFileRenamed(System.Object, MonoDevelop.Core.FileCopyEventArgs)
Source MonoDevelop.Core.EventQueue
00:02:10.0724091 - MonoDevelop.Projects.Project Void HandleFileChanged(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:18.8409343 - MonoDevelop.Ide.TypeSystem.MonoDevelopMetadataReference Void OnUpdatedOnDisk(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:15.7531517 - MonoDevelop.Projects.FileStatusTracker`1[MonoDevelop.Projects.SolutionItemEventArgs] Void HandleFileChanged(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:11.4172185 - MonoDevelop.Ide.Gui.Pads.ProjectPad.ShowAllFilesBuilderExtension Void OnSystemFileAdded(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:10.6727282 - MonoDevelop.Ide.TypeSystem.TypeSystemService Void FileService_FileChanged(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:02.6945364 - MonoDevelop.Unity.Project.NodeBuilders.UnityFileNodeBuilder Void OnFileChanged(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:01.6960784 - MonoDevelop.Projects.FileStatusTracker`1[MonoDevelop.Projects.WorkspaceItemEventArgs] Void HandleFileChanged(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:01.0363946 - MonoDevelop.PackageManagement.SdkProjectReloadMonitor Void FileChanged(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.5363695 - MonoDevelop.PackageManagement.NuGetConfigFileChangedMonitor Void FileChanged(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.4974428 - MonoDevelop.Ide.Gui.Pads.ProjectPad.ProjectNodeBuilder Void OnSystemFileCreated(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.4228951 - MonoDevelop.PackageManagement.NodeBuilders.ProjectPackagesProjectNodeBuilderExtension Void FileChanged(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.4220673 - MonoDevelop.Ide.Gui.DocumentRegistry Void HandleFileServiceChange(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.4020246 - MonoDevelop.PackageManagement.Commands.PackageManagementStartupHandler Void FileChanged(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.3517315 - MonoDevelop.Ide.TypeSystem.MonoDevelopAnalyzer Void OnUpdatedOnDisk(System.Object, System.EventArgs)
00:00:00.2972448 - MonoDevelop.Ide.Gui.Pads.ProjectPad.ShowAllFilesBuilderExtension Void OnSystemFileDeleted(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.1205992 - MonoDevelop.Ide.RootWorkspace Void CheckFileRemoved(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.0819609 - MonoDevelop.Projects.DotNetProject Void OnFileRemoved(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.0697519 - MonoDevelop.Ide.Editor.EditorConfigService+ConventionsFileManager Void FileService_FileChanged(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.0325364 - MonoDevelop.Unity.Project.NodeBuilders.UnityFileNodeBuilder Void OnFileCopied(System.Object, MonoDevelop.Core.FileCopyEventArgs)
00:00:00.0199356 - MonoDevelop.Ide.DesktopService Void NotifyFileRemoved(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.0117852 - MonoDevelop.Ide.Gui.Documents.DocumentManager Void CheckRemovedFile(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.0025223 - MonoDevelop.Ide.Gui.Pads.ProjectPad.ProjectNodeBuilder Void OnSystemFileDeleted(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.0024015 - MonoDevelop.Ide.Gui.Pads.ProjectPad.ShowAllFilesBuilderExtension Void OnSystemFileRenamed(System.Object, MonoDevelop.Core.FileCopyEventArgs)
00:00:00.0016862 - MonoDevelop.Ide.Editor.EditorConfigService+ConventionsFileManager Void FileService_FileRemoved(System.Object, MonoDevelop.Core.FileEventArgs)
00:00:00.0012199 - MonoDevelop.Ide.Gui.Pads.ProjectPad.ProjectFolder Void OnFileRenamed(System.Object, MonoDevelop.Core.FileCopyEventArgs)
00:00:00.0010056 - MonoDevelop.Debugger.DebuggingService Void FileService_FileRenamed(System.Object, MonoDevelop.Core.FileCopyEventArgs)
00:00:00.0009376 - MonoDevelop.Ide.Gui.Documents.DocumentManager Void CheckRenamedFile(System.Object, MonoDevelop.Core.FileCopyEventArgs)
00:00:00.0006533 - MonoDevelop.Ide.Editor.EditorConfigService+ConventionsFileManager Void FileService_FileMoved(System.Object, MonoDevelop.Core.FileCopyEventArgs)
00:00:00.0004185 - MonoDevelop.Ide.DesktopService Void NotifyFileRenamed(System.Object, MonoDevelop.Core.FileCopyEventArgs)
00:00:00.0003588 - MonoDevelop.Ide.RootWorkspace Void CheckFileRename(System.Object, MonoDevelop.Core.FileCopyEventArgs)
```

2 files changed, 35 insertions, 12 deletions

diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/DotNetProject.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/DotNetProject.cs
index cf85618ebe..d103fe3864 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/DotNetProject.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/DotNetProject.cs
@@ -548,16 +548,28 @@ namespace MonoDevelop.Projects
 			get { return LanguageBinding.SupportsPartialTypes; }
 		}
 
-		void CheckReferenceChange (FilePath updatedFile)
+		void CheckReferenceChange (FileEventArgs updatedFiles)
 		{
-			for (int n=0; n<References.Count; n++) {
+			int done = 0;
+			foreach (var item in updatedFiles) {
+				// HACK: Don't like this, but we need it for now.
+				if (!item.FileName.HasExtension ("dll"))
+					done++;
+			}
+
+			for (int n=0; n<References.Count && done != updatedFiles.Count; n++) {
 				ProjectReference pr = References [n];
 				if (pr.ReferenceType == ReferenceType.Assembly && DefaultConfiguration != null) {
-					if (pr.GetReferencedFileNames (DefaultConfiguration.Selector).Any (f => f == updatedFile)) {
-						SetFastBuildCheckDirty ();
-						pr.NotifyStatusChanged ();
+					foreach (var fileName in pr.GetReferencedFileNames (DefaultConfiguration.Selector)) {
+						if (Contains (updatedFiles, fileName)) {
+							done++;
+							SetFastBuildCheckDirty ();
+							pr.NotifyStatusChanged ();
+							break;
+						}
 					}
-				} else if (pr.HintPath == updatedFile) {
+				} else if (Contains (updatedFiles, pr.HintPath)) {
+					done++;
 					SetFastBuildCheckDirty ();
 					var nr = pr.GetRefreshedReference ();
 					if (nr != null)
@@ -565,6 +577,19 @@ namespace MonoDevelop.Projects
 				}
 			}
 
+			static bool Contains (FileEventArgs args, FilePath fileName)
+			{
+				if (fileName.IsNullOrEmpty)
+					return false;
+
+				foreach (var arg in args) {
+					if (arg.FileName == fileName)
+						return true;
+				}
+
+				return false;
+			}
+
 			// If a referenced assembly changes, dirtify the project.
 /*			if (DefaultConfiguration != null) {
 				foreach (var asm in await GetReferencedAssemblies (DefaultConfiguration.Selector))
@@ -587,8 +612,7 @@ namespace MonoDevelop.Projects
 				return;
 
 			base.OnFileChanged (source, e);
-			foreach (FileEventInfo ei in e)
-				CheckReferenceChange (ei.FileName);
+			CheckReferenceChange (e);
 		}
 
 
@@ -1874,10 +1898,9 @@ namespace MonoDevelop.Projects
 		/// </summary>
 		public event EventHandler ReferencedAssembliesChanged;
 
-		private void OnFileRemoved (Object o, FileEventArgs e)
+		private void OnFileRemoved (object o, FileEventArgs e)
 		{
-			foreach (FileEventInfo ei in e)
-				CheckReferenceChange (ei.FileName);
+			CheckReferenceChange (e);
 		}
 
 		protected async override Task OnExecute (ProgressMonitor monitor, ExecutionContext context, ConfigurationSelector configuration, SolutionItemRunConfiguration runConfiguration)
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/ProjectReference.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/ProjectReference.cs
index a77dbf985b..4493d8c6f8 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/ProjectReference.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/ProjectReference.cs
@@ -625,7 +625,7 @@ namespace MonoDevelop.Projects
 			
 			if (s != null)
 				return new string[] { s };
-			return new string [0];
+			return Array.Empty<string> ();
 		}
 		/*
 		void AddRequiredPackages (List<string> result, SystemPackage fromPackage)