Only enable the Dart timeline for benchmarks

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/Dart/initialized'
       },
       {
         'name': 'accept connection',
         'spec': 'time_between/Dart/initialized/Dart/connected',
       }
     ],
     'shell-args': [
-      '--args-for=mojo:dart_content_handler --complete-timeline',
+      '--args-for=mojo:dart_content_handler --dart-timeline',
     ]
   },
   {
     'name': 'dart init with handler',
     'app': 'https://core.mojoapps.io/dart_handler_running.mojo',
     'duration': 10,
     'measurements': [
       {
         'name': 'initialize',
         'spec': 'time_between/Dart/connecting/Dart/initialized',
       }
     ],
     'shell-args': [
-      '--args-for=mojo:dart_content_handler --complete-timeline',
+      '--args-for=mojo:dart_content_handler --dart-timeline',
     ]
   },
   {
     'name': 'dart init no observatory',
     'app': 'https://core.mojoapps.io/dart_startup.mojo',
     'duration': 10,
     'measurements': [
       {
         'name': 'initialize',
         'spec': 'time_until/Dart/initialized'
       },
       {
         'name': 'accept connection',
         'spec': 'time_between/Dart/initialized/Dart/connected',
       }
     ],
     'shell-args': [
-      '--args-for=mojo:dart_content_handler --disable-observatory --complete-timeline',
+      '--args-for=mojo:dart_content_handler --disable-observatory --dart-timeline',
     ]
   },
   {
     'name': 'dart init run (message loop)',
     'app': 'https://core.mojoapps.io/dart_startup.mojo',
     'duration': 10,
     'measurements': [
       {
         'name': 'initialize',
         'spec': 'time_until/Dart/initialized'
       },
       {
         'name': 'accept connection',
         'spec': 'time_between/Dart/initialized/Dart/connected',
       }
     ],
     'shell-args': [
-      '--args-for=mojo:dart_content_handler --run-on-message-loop --complete-timeline',
+      '--args-for=mojo:dart_content_handler --run-on-message-loop --dart-timeline',
     ]
   },
   {
     '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',
     ]
   },
 ]