Fast path Dart Timeline events with no arguments to avoid calling JSON.encode

runtime/observatory/tests/service/get_vm_timeline_rpc_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // VMOptions=--error_on_bad_type --error_on_bad_override --complete_timeline
 
 import 'dart:developer';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
 import 'test_helper.dart';
 
 primeTimeline() {
   Timeline.startSync('apple');
-  Timeline.instantSync('ISYNC');
+  Timeline.instantSync('ISYNC', arguments: {'fruit': 'banana'});
   Timeline.finishSync();
   TimelineTask task = new TimelineTask();
   task.start('TASK1');
   task.instant('ITASK');
   task.finish();
 }
 
 List<Map> filterForDartEvents(List<Map> events) {
   return events.where((event) => event['cat'] == 'Dart').toList();
 }
 
 bool eventsContains(List<Map> events, String phase, String name) {
   for (Map event in events) {
     if ((event['ph'] == phase) && (event['name'] == name)) {
       return true;
     }
   }
   return false;
 }
 
+void allEventsHaveIsolateNumber(List<Map> events) {
+  for (Map event in events) {
+    if (event['ph'] == 'M') {
+      // Skip meta-data events.
+      continue;
+    }
+    if (event['name'] == 'Runnable' && event['ph'] == 'i') {
+      // Skip Runnable events which don't have an isolate.
+      continue;
+    }
+    if (event['cat'] == 'VM') {
+      // Skip VM category events which don't have an isolate.
+      continue;
+    }
+    Map arguments = event['args'];
+    expect(arguments, new isInstanceOf<Map>());
+    expect(arguments['isolateNumber'], new isInstanceOf<String>());
+  }
+}
+
 var tests = [
   (VM vm) async {
     Map result = await vm.invokeRpcNoUpgrade('_getVMTimeline', {});
     expect(result['type'], equals('_Timeline'));
     expect(result['traceEvents'], new isInstanceOf<List>());
     List<Map> dartEvents = filterForDartEvents(result['traceEvents']);
     expect(dartEvents.length, equals(5));
+    allEventsHaveIsolateNumber(dartEvents);
+    allEventsHaveIsolateNumber(result['traceEvents']);
     expect(eventsContains(dartEvents, 'I', 'ISYNC'), isTrue);
     expect(eventsContains(dartEvents, 'X', 'apple'), isTrue);
     expect(eventsContains(dartEvents, 'b', 'TASK1'), isTrue);
     expect(eventsContains(dartEvents, 'e', 'TASK1'), isTrue);
     expect(eventsContains(dartEvents, 'n', 'ITASK'), isTrue);
     expect(eventsContains(dartEvents, 'q', 'ITASK'), isFalse);
   },
 ];
 
 main(args) async => runVMTests(args, tests, testeeBefore: primeTimeline);