Fix asserts in StackFrameIterator which were effectively disabled

runtime/vm/os_fuchsia.cc

 // Copyright (c) 2016, 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/globals.h"
 #if defined(HOST_OS_FUCHSIA)
 
 #include "vm/os.h"
 
 #include <errno.h>
@@ skipping 128 matching lines @@
   // TODO(5411554): Allow overriding default code alignment for
   // testing purposes.
   // Flags::DebugIsInt("codealign", &alignment);
   ASSERT(Utils::IsPowerOfTwo(alignment));
   ASSERT(alignment >= kMinimumAlignment);
   ASSERT(alignment <= OS::kMaxPreferredCodeAlignment);
   return alignment;
 }
 
 
-bool OS::AllowStackFrameIteratorFromAnotherThread() {
-  UNIMPLEMENTED();
-  return false;
-}
-
-
 int OS::NumberOfAvailableProcessors() {
   return sysconf(_SC_NPROCESSORS_CONF);
 }
 
 
 uintptr_t OS::MaxRSS() {
   mx_info_task_stats_t task_stats;
   mx_handle_t process = mx_process_self();
   mx_status_t status = mx_object_get_info(
       process, MX_INFO_TASK_STATS, &task_stats, sizeof(task_stats), NULL, NULL);
@@ skipping 150 matching lines @@
 }
 
 
 void OS::Exit(int code) {
   UNIMPLEMENTED();
 }
 
 }  // namespace dart
 
 #endif  // defined(HOST_OS_FUCHSIA)