Added custom tags helper classes to the Observatory

runtime/observatory/lib/src/elements/helpers/rendering_queue.dart

Index: runtime/observatory/lib/src/elements/helpers/rendering_queue.dart
diff --git a/runtime/observatory/lib/src/elements/helpers/rendering_queue.dart b/runtime/observatory/lib/src/elements/helpers/rendering_queue.dart
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index 0000000000000000000000000000000000000000..a6062c38ebd64201b88518d6397e5c505f15d19b
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+++ b/runtime/observatory/lib/src/elements/helpers/rendering_queue.dart
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+// Copyright (c) 2016, 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.
+
+import 'dart:html';
+import 'dart:collection';
+import 'dart:async';
+
+/// A generic rendering task that can be scheduled.
+abstract class RenderingTask {
+  /// Rendering synchronous callback.
+  void render();
+}
+
+/// A generic synchronization system for rendering operations.
+abstract class RenderingBarrier {
+  /// Future to the next synchronization barrier (ms from application start).
+  Future<num> get next;
+}
+
+/// Synchronization system based on the AnimationFrame.
+class NextAnimationFrameBarrier implements RenderingBarrier {
+  Future<num> get next => window.animationFrame;
+}
+
+/// MOCK synchronization system for manual barrier triggering.
+class RenderingBarrierMock implements RenderingBarrier {
+  final StreamController<num> _stream = new StreamController<num>.broadcast();
+  num _ms = 0;
+
+  Future<num> get next => _stream.stream.first;
+
+  /// Trigger the next barrier with an optional numer of ms elapsed.
+  void triggerRenderingBarrier({num step: 20}) {
+    assert(step != null);
+    _stream.add(_ms += step);
+  }
+}
+
+/// RenderingTask queuing and synchronization system.
+class RenderingQueue {
+  final RenderingBarrier _barrier;
+  final Queue<RenderingTask> _queue = new Queue<RenderingTask>();
+
+  bool get isEmpty => _queue.isEmpty;
+  bool get isNotEmpty => _queue.isNotEmpty;
+
+  /// Creates a RenderingQueue with the default synchronization barrier.
+  RenderingQueue() : this.fromBarrier(new NextAnimationFrameBarrier());
+
+  /// Creates a RenderingQueue with a custom synchronization barrier.
+  RenderingQueue.fromBarrier(this._barrier) {
+    assert(this._barrier != null);
+  }
+
+  /// Add a task to the queue.
+  /// If the current rendering phase is running it will be executed during this
+  /// rendering cycle, otherwise it will be queued for the next one.
+  void enqueue(RenderingTask r) {
+    assert(r != null);
+    // If no task are in the queue there is no rendering phase scheduled.
+    if (isEmpty) _render();
+    _queue.addLast(r);
+  }
+
+  Future _render() async {
+    await _barrier.next;
+    while (_queue.isNotEmpty) {
+      _queue.removeFirst().render();
+    }
+  }
+}