fix spelling in core libraries

sdk/lib/core/stopwatch.dart

Index: sdk/lib/core/stopwatch.dart
diff --git a/sdk/lib/core/stopwatch.dart b/sdk/lib/core/stopwatch.dart
index f6dd85e471c59106710183571fb6362f41828f82..cf55b89d1830ab1661478eb112d6d4ea208f7a3b 100644
--- a/sdk/lib/core/stopwatch.dart
+++ b/sdk/lib/core/stopwatch.dart
@@ -15,7 +15,7 @@ class Stopwatch {
 
   // The _start and _stop fields capture the time when [start] and [stop]
   // are called respectively.
-  // If _stop is null, the stopwatched is running.
+  // If _stop is null, the stopwatch is running.
   int _start = 0;
   int _stop = 0;