Merge pull request #1218 from AndreyAkinshin/master

Suppression of negative Elapsed in Stopwatch, this is the comment from the Reference Source:

                    // When measuring small time periods the StopWatch.Elapsed*
                    // properties can return negative values.  This is due to
                    // bugs in the basic input/output system (BIOS) or the hardware
                    // abstraction layer (HAL) on machines with variable-speed CPUs
                    // (e.g. Intel SpeedStep).

mcs/class/System/System.Diagnostics/Stopwatch.cs

@@ -113,6 +113,8 @@ public void Stop ()
 			if (!is_running)
 				return;
 			elapsed += GetTimestamp () - started;
+			if (elapsed < 0)
+				elapsed = 0;
 			is_running = false;
 		}