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

mojo/tools/data/benchmarks

 # This file contains a list of Mojo benchmarks. For description of the file
 # format, see `mojo_benchmark --help`.
 
 benchmarks = [
   {
     'name': 'dart init',
     'app': 'https://core.mojoapps.io/dart_startup.mojo',
     'duration': 10,
     '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',
       }
     ]
   },
   {
     'name': 'dart init with handler',
     'app': 'https://core.mojoapps.io/dart_handler_running.mojo',
     'duration': 10,
     'measurements': [
       {
         'name': 'initialize',
-        'spec': 'time_between/dart_handler_running/connecting/traced_application/initialized',
+        'spec': 'time_between/Dart/connecting/Dart/initialized',
       }
     ]
   },
   {
     'name': 'cpp init',
     'app': 'https://core.mojoapps.io/trace_me.mojo',
     'duration': 10,
     'measurements': [
       {
         'name': 'initialize',
         'spec': 'time_until/trace_me/initialized',
       },
       {
         'name': 'accept connection',
         'spec': 'time_between/trace_me/initialized/trace_me/connected',
       }
     ],
     'shell-args': [
       '--args-for=https://core.mojoapps.io/trace_me.mojo --early-tracing',
     ]
   },
 ]