Fix compiler stats timer code

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 3438)
+++ runtime/vm/object.cc	(working copy)
@@ -3671,14 +3671,11 @@
   }
 
   // Get the source, scan and allocate the token stream.
-  if (FLAG_compiler_stats) {
-    CompilerStats::scanner_timer.Start();
-  }
+  TimerScope timer(FLAG_compiler_stats, &CompilerStats::scanner_timer);
   const String& src = String::Handle(source());
   Scanner scanner(src, private_key);
   set_tokens(TokenStream::Handle(TokenStream::New(scanner.GetStream())));
   if (FLAG_compiler_stats) {
-    CompilerStats::scanner_timer.Stop();
     CompilerStats::src_length += src.Length();
   }
 }