VM: Teach GetAndValidateIsolateStackBounds(...) to fallback to OS thread stack bounds.

runtime/vm/os_thread_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 "platform/globals.h"  // NOLINT
 #if defined(TARGET_OS_FUCHSIA)
 
 #include "vm/os_thread.h"
 #include "vm/os_thread_fuchsia.h"
 
@@ skipping 185 matching lines @@
 ThreadId OSThread::ThreadIdFromIntPtr(intptr_t id) {
   return static_cast<ThreadId>(id);
 }
 
 
 bool OSThread::Compare(ThreadId a, ThreadId b) {
   return pthread_equal(a, b) != 0;
 }
 
 
+bool OSThread::GetCurrentStackBounds(uword* lower, uword* upper) {
+  return false;
+}
+
+
 Mutex::Mutex() {
   pthread_mutexattr_t attr;
   int result = pthread_mutexattr_init(&attr);
   VALIDATE_PTHREAD_RESULT(result);
 
 #if defined(DEBUG)
   result = pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK);
   VALIDATE_PTHREAD_RESULT(result);
 #endif  // defined(DEBUG)
 
@@ skipping 203 matching lines @@
 void Monitor::NotifyAll() {
   // When running with assertions enabled we track the owner.
   ASSERT(IsOwnedByCurrentThread());
   int result = pthread_cond_broadcast(data_.cond());
   VALIDATE_PTHREAD_RESULT(result);
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_FUCHSIA)