Adding some comments to Timer.

AsyncRx.NET/System.Reactive.Async/System/Reactive/Linq/Operators/Timer.cs

@@ -128,6 +128,8 @@ namespace System.Reactive.Linq
             {
                 ct.ThrowIfCancellationRequested();
 
+                // TODO: Compensate for drift by adding stopwatch functionality.
+
                 do
                 {
                     await observer.OnNextAsync(tick++).RendezVous(scheduler);
@@ -154,6 +156,8 @@ namespace System.Reactive.Linq
             {
                 ct.ThrowIfCancellationRequested();
 
+                // TODO: Compensate for drift by adding stopwatch functionality.
+
                 do
                 {
                     await observer.OnNextAsync(tick++).RendezVous(scheduler);