Further fix get_cpu_profile_timeline_rpc_test.

runtime/observatory/tests/service/get_cpu_profile_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
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
 import 'test_helper.dart';
 
@@ skipping 30 matching lines @@
         expect(frames.containsKey(frame['parent']), isTrue);
       }
     }
 
     List events = result['traceEvents'];
     for (Map event in events) {
       expect(event['ph'], equals('P'));
       expect(event['pid'], isInt);
       expect(event['tid'], isInt);
       expect(event['ts'], isInt);
-      expect(event['category'], equals("Dart"));
+      expect(event['cat'], equals("Dart"));
       expect(frames.containsKey(event['sf']), isTrue);
     }
   },
 ];
 
 main(args) async => runIsolateTests(args, tests, testeeBefore: testeeDo);