Add request tracing to Observatory:

runtime/bin/vmservice/client/lib/tracer.dart

+// Copyright (c) 2013, 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.
+
+library tracer;
+
+import 'dart:async';
+import 'dart:html';
+
+import 'package:logging/logging.dart';
+import 'package:observe/observe.dart';
+
+_deepCopy(src) {
+  if (src is Map) {
+    var dest = {};
+    src.forEach((key, val) {
+        dest[key] = _deepCopy(val);
+    });
+    return dest;
+  } else if (src is List) {
+    var dest = [];
+    src.forEach((val) {
+        dest.add(_deepCopy(val));
+    });
+    return dest;
+  } else {
+    return src;
+  }
+}
+
+class TraceEvent {
+  TraceEvent.msg(this._time, this.message, Map originalMap) {
+    map = _deepCopy(originalMap);
+  }
+
+  String get timeStamp => "T+${_time.toStringAsFixed(3)}";
+
+  String toString() {
+    return "[${timeStamp}] ${message}";
+  }
+
+  double _time;
+  String message;
+  Map map;
+}
+
+class Tracer extends Observable {
+  // The current global tracer.
+  static Tracer get current => _current;
+
+  static Tracer _current;
+
+  static void start() {
+    if (_current == null) {
+      _current = new Tracer();
+    }
+  }
+  static void stop() {
+    _current = null;
+  }
+  
+  // The tracer subscribes to all logging events.
+  StreamSubscription loggerSub = null;
+
+  // The start time for the current request.
+  double _startTime;
+
+  // A list of all tracing events for thre current request.
+  ObservableList<TraceEvent> events = new ObservableList<TraceEvent>();
+
+  Tracer() {
+    loggerSub = Logger.root.onRecord.listen((LogRecord rec) {
+        // Echo all log messages to the trace.
+        trace('${rec.level.name}: ${rec.message}');
+      });

[Cutch, 2014/07/30 18:20:46]

Should we cancel this subscription in stop()?

[turnidge, 2014/07/30 19:28:15]

Done.

+    reset();
+  }
+
+  void reset() {
+    _startTime = window.performance.now();

[Cutch, 2014/07/30 18:20:46]

If you use a Stopwatch you could not depend on dart:html - right?

[turnidge, 2014/07/30 19:28:15]

Done.

+    events.clear();
+  }
+
+  TraceEvent trace(String message, {Map map: null}) {
+    double now = window.performance.now();
+    var event = new TraceEvent.msg((now - _startTime), message, map);
+    events.add(event);
+    return event;
+  } 
+}