Add support for emitting asynchronous timeline events from Dart code

sdk/lib/developer/timeline.dart

Index: sdk/lib/developer/timeline.dart
diff --git a/sdk/lib/developer/timeline.dart b/sdk/lib/developer/timeline.dart
index 54c5b80f5ae89492bc667a170ee6d42f5a06b183..c42e5c0052bc0cce81f854ab901ff45b0ab4ab5d 100644
--- a/sdk/lib/developer/timeline.dart
+++ b/sdk/lib/developer/timeline.dart
@@ -32,8 +32,8 @@ class Timeline {
     }
     // Pop top item off of stack.
     var block = _stack.removeLast();
-    // Close it.
-    block.close();
+    // Finish it.
+    block.finish();
   }
 
   /// A utility method to time a synchronous [function]. Internally calls
@@ -52,6 +52,130 @@ class Timeline {
   static final List<_SyncBlock> _stack = new List<_SyncBlock>();
 }
 
+/// An asynchronous task on the timeline. Asynchronous tasks can live
+/// longer than the current event and can even be shared between isolates.
+/// An asynchronous task can have many (nested) blocks. To share a
+/// [TimelineTask] across isolates, you must construct a [TimelineTask] in
+/// both isolates using the same [taskId] and [category].
+class TimelineTask {
+  /// Create a task. [taskId] will be set by the system.
+  /// Optionally you can specify a [category] name.
+  TimelineTask({String category: 'Dart'})
+      : _taskId = _getNextAsyncId(),
+        category = category {
+    if (category is! String) {
+      throw new ArgumentError.value(category,
+                                    'category',
+                                    'Must be a String');
+    }
+  }
+
+  /// Create a task with an explicit [taskId]. This is useful if you are
+  /// passing a task between isolates. Optionally you can specify a [category]
+  /// name.
+  TimelineTask.withTaskId(int taskId, {String category: 'Dart'})
+      : _taskId = taskId,
+        category = category {
+    if (taskId is! int) {
+      throw new ArgumentError.value(taskId,
+                                    'taskId',
+                                    'Must be an int');
+    }
+    if (category is! String) {
+      throw new ArgumentError.value(category,
+                                    'category',
+                                    'Must be a String');
+    }
+  }
+
+  /// Start a block in this task named [name]. Optionally takes
+  /// a [Map] of [arguments].
+  /// Returns an [AsyncBlock] which is used to finish this block.
+  AsyncBlock start(String name, {Map arguments}) {
+    if (name is! String) {
+      throw new ArgumentError.value(name,
+                                    'name',
+                                    'Must be a String');
+    }
+    var block = new AsyncBlock._(name, _taskId, category);
+    if (arguments is Map) {
+      block.arguments.addAll(arguments);
+    }
+    /// Emit start event.
+    block._start();
+    return block;
+  }
+
+  /// Retrieve the asynchronous task's id. Can be used to construct a
+  /// [TimelineTask] in another isolate.
+  int get taskId => _taskId;
+  final int _taskId;
+  /// Retrieve the asynchronous task's category. Can be used to construct a
+  /// [TimelineTask] in another isolate.
+  final String category;
+}
+
+/// An asynchronous block of time on the timeline. This block can be kept
+/// open across isolate messages.
+class AsyncBlock {
+  /// The category this block belongs to.
+  final String category;
+  /// The name of this block.
+  final String name;
+  /// The asynchronous task id.
+  final int _taskId;
+  /// An (optional) set of arguments which will be serialized to JSON and
+  /// associated with this block.
+  final Map arguments = {};
+  bool _finished = false;
+
+  AsyncBlock._(this.name, this._taskId, this.category);
+
+  // Emit the start event.
+  void _start() {
+    String argumentsAsJson = JSON.encode(arguments);
+    _reportTaskEvent(_getTraceClock(),
+                     _taskId,
+                     'b',
+                     category,
+                     name,
+                     argumentsAsJson);
+  }
+
+  // Emit the finish event.
+  void _finish() {
+    _reportTaskEvent(_getTraceClock(),
+                     _taskId,
+                     'e',
+                     category,
+                     name,
+                     JSON.encode({}));
+  }
+
+  /// Finish this block. Cannot be called twice.
+  void finish() {
+    if (_finished) {
+      throw new StateError(
+          'It is illegal to call finish twice on the same AsyncBlock');
+    }
+    _finished = true;
+    _finish();
+  }
+
+  /// Finishes this block when [future] completes. Returns a [Future]
+  /// chained to [future].
+  Future finishWhenComplete(Future future) {
+    if (future is! Future) {
+      throw new ArgumentError.value(future,
+                                    'future',
+                                    'Must be a Future');
+    }
+    return future.whenComplete(() {
+      finish();
+    });
+  }
+}
+
 /// A synchronous block of time on the timeline. This block should not be
 /// kept open across isolate messages.
 class _SyncBlock {
@@ -64,20 +188,13 @@ class _SyncBlock {
   final Map arguments = {};
   // The start time stamp.
   final int _start;
-  // Has this block been closed?
-  bool _closed = false;
 
   _SyncBlock._(this.name,
                this._start);
 
-  /// Close this block of time. At this point, this block can no longer be
+  /// Finish this block of time. At this point, this block can no longer be
   /// used.
-  void close() {
-    if (_closed) {
-      throw new StateError(
-          'It is illegal to call close twice on the same _SyncBlock');
-    }
-    _closed = true;
+  void finish() {
     var end = _getTraceClock();
 
     // Encode arguments map as JSON before reporting.
@@ -92,9 +209,20 @@ class _SyncBlock {
   }
 }
 
+/// Returns the next async task id.
+external int _getNextAsyncId();
+
 /// Returns the current value from the trace clock.
 external int _getTraceClock();
 
+/// Reports an event for a task.
+external void _reportTaskEvent(int start,
+                               int taskId,
+                               String phase,
+                               String category,
+                               String name,
+                               String argumentsAsJson);
+
 /// Reports a complete synchronous event.
 external void _reportCompleteEvent(int start,
                                    int end,