Add ability to save and load timelines from Observatory

runtime/observatory/lib/src/elements/timeline_page.dart

Index: runtime/observatory/lib/src/elements/timeline_page.dart
diff --git a/runtime/observatory/lib/src/elements/timeline_page.dart b/runtime/observatory/lib/src/elements/timeline_page.dart
index fbecb109fc8b53fc0bd31cf9501d2007af773359..7a70c1102dc387189ecf4492f6b2bd3f43e9ab25 100644
--- a/runtime/observatory/lib/src/elements/timeline_page.dart
+++ b/runtime/observatory/lib/src/elements/timeline_page.dart
@@ -67,6 +67,14 @@ class TimelinePageElement extends ObservatoryElement {
     });
   }
 
+  Future saveTimeline() async {
+    return postMessage('save');
+  }
+
+  Future loadTimeline() async {
+    return postMessage('load');
+  }
+
   _updateSize() {
     IFrameElement e = $['root'];
     final totalHeight = window.innerHeight;