it doesn't have to be high resolution, but I need to find Stopwatch in System.Diagnostics in Silverlight 5. In WP7, it is in Microsoft.Phone. (Sharing code across multiple platforms.)
I think it does exist, perhaps formerly in Microsoft.Device.dll.
Update Temporarily, it seems work to pull the Stopwatch.cs from the Mono library (which ultimately maps Stopwatch to Timespan).
Update2 Nope, that doesn't work, since it tries to call extern long GetTimestamp(); Probably this needs to be replaced with something SL can handle. Possibility http://blog.tiaan.com/link/2009/02/03/stopwatch-silverlight