Use Properties indexer instead of Add() to track pasted span

src/EditorFeatures/CSharpTest/CodeRefactorings/AddMissingImports/CSharpAddMissingImportsRefactoringProviderTests.cs

@@ -36,6 +36,10 @@ protected override TestWorkspace CreateWorkspaceFromFile(string initialMarkup, T
             if (!pastedTextSpan.IsEmpty)
             {
                 var pasteTrackingService = workspace.ExportProvider.GetExportedValue<PasteTrackingService>();
+
+                // This tests the paste tracking service's resiliancy to failing when multiple pasted spans are
+                // registered consecutively and that the last registered span wins.
+                pasteTrackingService.RegisterPastedTextSpan(hostDocument.TextBuffer, default);
                 pasteTrackingService.RegisterPastedTextSpan(hostDocument.TextBuffer, pastedTextSpan);
             }
 

src/EditorFeatures/Core/Implementation/PasteTracking/PasteTrackingService.cs

@@ -44,7 +44,7 @@ internal void RegisterPastedTextSpan(ITextBuffer textBuffer, TextSpan textSpan)
             // Any change to the TextBuffer will remove the pasted text span.
             // This includes consecutive paste operations which will fire the
             // Changed event prior to the handler registering a new text span.
-            textBuffer.Properties.AddProperty(_pastedTextSpanKey, textSpan);
+            textBuffer.Properties[_pastedTextSpanKey] = textSpan;
             textBuffer.Changed += RemovePastedTextSpan;
 
             return;

src/EditorFeatures/VisualBasicTest/CodeRefactorings/AddMissingImports/VisualBasicAddMissingImportsRefactoringProviderTests.vb

@@ -28,6 +28,10 @@ Namespace Microsoft.CodeAnalysis.AddMissingImports
 
             If Not pastedTextSpan.IsEmpty Then
                 Dim PasteTrackingService = Workspace.ExportProvider.GetExportedValue(Of PasteTrackingService)()
+
+                ' This tests the paste tracking service's resiliancy to failing when multiple pasted spans are
+                ' registered consecutively And that the last registered span wins.
+                PasteTrackingService.RegisterPastedTextSpan(hostDocument.TextBuffer, Nothing)
                 PasteTrackingService.RegisterPastedTextSpan(hostDocument.TextBuffer, pastedTextSpan)
             End If