Add ThreadBarrier; use in GCMarker and unit test

runtime/vm/random.h

Index: runtime/vm/random.h
diff --git a/runtime/vm/random.h b/runtime/vm/random.h
index cda8dd87d60dcae56addbffbb2071edfee45bdb2..f5614dedbfce29854178f3c1665f379a088d5da3 100644
--- a/runtime/vm/random.h
+++ b/runtime/vm/random.h
@@ -13,6 +13,8 @@ namespace dart {
 class Random {
  public:
   Random();
+  // Seed must be non-zero.
+  explicit Random(uint64_t seed);
   ~Random();
 
   uint32_t NextUInt32();
@@ -23,6 +25,7 @@ class Random {
 
  private:
   void NextState();
+  void Initialize(uint64_t seed);
 
   uint64_t _state;