Introduce simple Thread class to support gradual refactoring of interfaces.

runtime/vm/bit_vector.h

 // Copyright (c) 2012, 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_BIT_VECTOR_H_
 #define VM_BIT_VECTOR_H_
 
 #include "vm/allocation.h"
 #include "vm/isolate.h"
 #include "vm/zone.h"
@@ skipping 26 matching lines @@
 
    private:
     BitVector* target_;
     intptr_t bit_index_;
     intptr_t word_index_;
     uword current_word_;
 
     friend class BitVector;
   };
 
+  BitVector(Zone* zone, intptr_t length)
+      : length_(length),
+        data_length_(SizeFor(length)),
+        data_(zone->Alloc<uword>(data_length_)) {
+    Clear();
+  }
+
+  // DEPRECATED: Use Zone version.
   BitVector(Isolate* isolate, intptr_t length)
       : length_(length),
         data_length_(SizeFor(length)),
         data_(isolate->current_zone()->Alloc<uword>(data_length_)) {
     Clear();
   }
 
   void CopyFrom(const BitVector* other) {
     Clear();
     AddAll(other);
@@ skipping 53 matching lines @@
   intptr_t length_;
   intptr_t data_length_;
   uword* data_;
 
   DISALLOW_COPY_AND_ASSIGN(BitVector);
 };
 
 }  // namespace dart
 
 #endif  // VM_BIT_VECTOR_H_