Add Thread-based constructors to TimerScope, TimelineDurationScope.

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index 05e0ef04e90b6d6223231e6aaf41cf7a26006bf5..6a9889c93ba361c439f2b00d4bcc92bd318f5f75 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -219,7 +219,7 @@ class Heap {
     stats_.data_[id] = value;
   }
 
-  bool gc_in_progress() const { return gc_in_progress_; }
+  bool gc_in_progress();
 
   static bool IsAllocatableInNewSpace(intptr_t size) {
     return size <= kNewAllocatableSize;
@@ -297,6 +297,10 @@ class Heap {
   void UpdateClassHeapStatsBeforeGC(Heap::Space space);
   void UpdatePretenurePolicy();
 
+  // Updates gc_in_progress.
+  void BeginGC();
+  void EndGC();
+
   // If this heap is non-empty, updates start and end to the smallest range that
   // contains both the original [start, end) and the [lowest, highest) addresses
   // of this heap.
@@ -318,6 +322,7 @@ class Heap {
   bool read_only_;
 
   // GC on the heap is in progress.
+  Mutex gc_in_progress_mutex_;
   bool gc_in_progress_;
 
   int pretenure_policy_;