Copy event label.

runtime/vm/timeline.cc

Index: runtime/vm/timeline.cc
diff --git a/runtime/vm/timeline.cc b/runtime/vm/timeline.cc
index 68e0784d9317f17625fcf556ab4ad7dd58815751..9fd9e21c265420739aac4373c4be7dc816a7610b 100644
--- a/runtime/vm/timeline.cc
+++ b/runtime/vm/timeline.cc
@@ -299,7 +299,10 @@ void TimelineEvent::Reset() {
   thread_ = OSThread::kInvalidThreadId;
   isolate_id_ = ILLEGAL_PORT;
   category_ = "";
-  label_ = NULL;
+  if (label_ != NULL) {
+    free(label_);
+    label_ = NULL;
+  }
   FreeArguments();
   set_pre_serialized_json(false);
   set_event_type(kNone);
@@ -498,7 +501,7 @@ void TimelineEvent::Init(EventType event_type,
   } else {
     isolate_id_ = ILLEGAL_PORT;
   }
-  label_ = label;
+  label_ = strdup(label);

[Cutch, 2016/04/14 13:24:29]

The timeline was designed to require const char* for the label to avoid doing
this string copy. 

Flutter should give us const char* for the labels, or, change this CL to make
copying of the label optional and only use it for events delivered via the API.

   FreeArguments();
   set_pre_serialized_json(false);
   set_event_type(event_type);