Ensure TimelineEventRingRecorder outputs blocks in increasing time order

runtime/vm/timeline.cc

Index: runtime/vm/timeline.cc
diff --git a/runtime/vm/timeline.cc b/runtime/vm/timeline.cc
index d29b057d87380267231fe153c4830eb2cbccdc51..d2cb26cef0fe24204917450c541f5c673491651c 100644
--- a/runtime/vm/timeline.cc
+++ b/runtime/vm/timeline.cc
@@ -420,10 +420,14 @@ TimelineEventRingRecorder::~TimelineEventRingRecorder() {
 
 
 void TimelineEventRingRecorder::PrintJSONEvents(JSONArray* events) const {
-  // TODO(johnmccutchan): This output needs to start with the oldest block
-  // first.
+  intptr_t block_offset = FindOldestBlockIndex();
+  if (block_offset == -1) {
+    // All blocks are empty.
+    return;
+  }
   for (intptr_t block_idx = 0; block_idx < num_blocks_; block_idx++) {
-    TimelineEventBlock* block = blocks_[block_idx];
+    TimelineEventBlock* block =
+        blocks_[(block_idx + block_offset) % num_blocks_];
     if (block->IsEmpty()) {
       // Skip empty blocks.
       continue;
@@ -471,6 +475,24 @@ TimelineEventBlock* TimelineEventRingRecorder::GetNewBlockLocked() {
 }
 
 
+intptr_t TimelineEventRingRecorder::FindOldestBlockIndex() const {
+  int64_t earliest_time = kMaxInt64;
+  intptr_t earliest_index = -1;
+  for (intptr_t block_idx = 0; block_idx < num_blocks_; block_idx++) {
+    TimelineEventBlock* block = blocks_[block_idx];
+    if (block->IsEmpty()) {
+      // Skip empty blocks.
+      continue;
+    }
+    if (block->LowerTimeBound() < earliest_time) {
+      earliest_time = block->LowerTimeBound();
+      earliest_index = block_idx;
+    }
+  }
+  return earliest_index;
+}
+
+
 void TimelineEventRingRecorder::VisitObjectPointers(
     ObjectPointerVisitor* visitor) {
   visitor->VisitPointer(reinterpret_cast<RawObject**>(&event_objects_));