VM: Fix memory leaks during isolate spawning

runtime/vm/snapshot.cc

 // 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.
 
 #include "vm/snapshot.h"
 
 #include "platform/assert.h"
 #include "vm/bootstrap.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart.h"
@@ skipping 1890 matching lines @@
   for (RawObject** current = first; current <= last; current++) {
     RawObject* raw_obj = *current;
     writer_->WriteObjectImpl(raw_obj, as_references_);
   }
 }
 
 
 MessageWriter::MessageWriter(uint8_t** buffer,
                              ReAlloc alloc,
                              DeAlloc dealloc,
-                             bool can_send_any_object)
+                             bool can_send_any_object,
+                             intptr_t* buffer_len)
     : SnapshotWriter(Thread::Current(),
                      Snapshot::kMessage,
                      buffer,
                      alloc,
                      dealloc,
                      kInitialSize,
                      &forward_list_,
                      can_send_any_object),
-      forward_list_(thread(), kMaxPredefinedObjectIds) {
+      forward_list_(thread(), kMaxPredefinedObjectIds),
+      buffer_len_(buffer_len) {
   ASSERT(buffer != NULL);
   ASSERT(alloc != NULL);
 }
 
 
 void MessageWriter::WriteMessage(const Object& obj) {
   ASSERT(kind() == Snapshot::kMessage);
   ASSERT(isolate() != NULL);
 
   // Setup for long jump in case there is an exception while writing
   // the message.
   LongJumpScope jump;
   if (setjmp(*jump.Set()) == 0) {
     NoSafepointScope no_safepoint;
     WriteObject(obj.raw());
+    if (buffer_len_ != NULL) {
+      *buffer_len_ = BytesWritten();
+    }
   } else {
     FreeBuffer();
     ThrowException(exception_type(), exception_msg());
   }
 }
 
 
 }  // namespace dart