Dart: Use dart:developer for tracing in startup benchmark.

BUG=
R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org/1443433002 .
diff --git a/mojo/tools/data/benchmarks b/mojo/tools/data/benchmarks
index e48a3c5..b3fe877 100644
--- a/mojo/tools/data/benchmarks
+++ b/mojo/tools/data/benchmarks
@@ -9,11 +9,11 @@
     'measurements': [
       {
         'name': 'initialize',
-        'spec': 'time_until/traced_application/initialized'
+        'spec': 'time_until/Dart/initialized'
       },
       {
         'name': 'accept connection',
-        'spec': 'time_between/traced_application/initialized/traced_application/connected',
+        'spec': 'time_between/Dart/initialized/Dart/connected',
       }
     ]
   },
@@ -24,7 +24,7 @@
     'measurements': [
       {
         'name': 'initialize',
-        'spec': 'time_between/dart_handler_running/connecting/traced_application/initialized',
+        'spec': 'time_between/Dart/connecting/Dart/initialized',
       }
     ]
   },