VM: Fix memory leaks during isolate spawning

runtime/vm/datastream.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 RUNTIME_VM_DATASTREAM_H_
 #define RUNTIME_VM_DATASTREAM_H_
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/allocation.h"
@@ skipping 292 matching lines @@
     *buffer_ = reinterpret_cast<uint8_t*>(alloc_(NULL, 0, initial_size_));
     if (*buffer_ == NULL) {
       Exceptions::ThrowOOM();
     }
     current_ = *buffer_;
     current_size_ = initial_size_;
     end_ = *buffer_ + initial_size_;
   }
 
   uint8_t* buffer() const { return *buffer_; }
+  void set_buffer(uint8_t* value) { *buffer_ = value; }
   intptr_t bytes_written() const { return current_ - *buffer_; }
 
   void set_current(uint8_t* value) { current_ = value; }
 
   void Align(intptr_t alignment) {
     intptr_t position = current_ - *buffer_;
     position = Utils::RoundUp(position, alignment);
     current_ = *buffer_ + position;
   }
 
@@ skipping 133 matching lines @@
   intptr_t current_size_;
   ReAlloc alloc_;
   intptr_t initial_size_;
 
   DISALLOW_COPY_AND_ASSIGN(WriteStream);
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_DATASTREAM_H_