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).

diff --cc mcs/class/System/System.Diagnostics/Stopwatch.cs
Simple merge