[polymer] Merge of master into polymer10-migration

tracing/tracing/model/event_set.html

Index: tracing/tracing/model/event_set.html
diff --git a/tracing/tracing/model/event_set.html b/tracing/tracing/model/event_set.html
index c79ddc85302d780c6a385d43ce176fa1da52027a..ad3b5f8520be7b94df31c1be49f433172e232dd1 100644
--- a/tracing/tracing/model/event_set.html
+++ b/tracing/tracing/model/event_set.html
@@ -190,21 +190,14 @@ tr.exportTo('tr.model', function() {
       });
     },
 
-    getEventsOrganizedByCallback: function(cb) {
-      var eventsByCallback = {};
-      for (var i = 0; i < this.length; i++) {
-        var event = this[i];
-        var key = cb(event);
-
-        if (key === undefined)
-          throw new Error('An event could not be organized');
-
-        if (eventsByCallback[key] === undefined)
-          eventsByCallback[key] = new EventSet();
-
-        eventsByCallback[key].push(event);
-      }
-      return eventsByCallback;
+    /**
+     * @param {!function(!tr.model.Event):string} cb
+     * @param {*=} opt_this
+     * @return {!Object}
+     */
+    getEventsOrganizedByCallback: function(cb, opt_this) {
+      var groupedEvents = tr.b.group(this, cb, opt_this || this);
+      return tr.b.mapItems(groupedEvents, (_, events) => new EventSet(events));
     },
 
     enumEventsOfType: function(type, func) {