Rename StopWatch to Stopwatch. Add new Stopwatch.start() and Stopwatch.reset().

frog/world.dart

Index: frog/world.dart
diff --git a/frog/world.dart b/frog/world.dart
index 5b652e45f55af552f62c904b32d4bcb58caf8635..a6a8bffe57325977db32072ce7fc016f7c92d083 100644
--- a/frog/world.dart
+++ b/frog/world.dart
@@ -433,7 +433,7 @@ class World {
   }
 
   withTiming(String name, f()) {
-    final sw = new StopWatch();
+    final sw = new Stopwatch();
     sw.start();
     var result = f();
     sw.stop();