Add ThreadBarrier; use in GCMarker and unit test

runtime/vm/random.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_RANDOM_H_
 #define VM_RANDOM_H_
 
 #include "vm/globals.h"
 #include "vm/allocation.h"
 
 namespace dart {
 
 class Random {
  public:
   Random();
+  // Seed must be non-zero.
+  explicit Random(uint64_t seed);
   ~Random();
 
   uint32_t NextUInt32();
   uint64_t NextUInt64() {
     return (static_cast<uint64_t>(NextUInt32()) << 32) |
         static_cast<uint64_t>(NextUInt32());
   }
 
  private:
   void NextState();
+  void Initialize(uint64_t seed);
 
   uint64_t _state;
 
   DISALLOW_COPY_AND_ASSIGN(Random);
 };
 
 }  // namespace dart
 
 #endif  // VM_RANDOM_H_