Fuchsia: Fixes for Debug build

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(TARGET_OS_FUCHSIA)
 
 #include "vm/os.h"
 
 #include <magenta/syscalls.h>
 #include <magenta/types.h>
 
 #include "platform/assert.h"
 #include "vm/zone.h"
 
 namespace dart {
 
+#ifndef PRODUCT
+
+DEFINE_FLAG(bool, generate_perf_events_symbols, false,
+    "Generate events symbols for profiling with perf");
+
+#endif  // !PRODUCT
+
 const char* OS::Name() {
   return "fuchsia";
 }
 
 
 intptr_t OS::ProcessId() {
   UNIMPLEMENTED();
   return 0;
 }
 
@@ skipping 204 matching lines @@
 }
 
 
 bool OS::StringToInt64(const char* str, int64_t* value) {
   UNIMPLEMENTED();
   return false;
 }
 
 
 void OS::RegisterCodeObservers() {
-  UNIMPLEMENTED();
+#ifndef PRODUCT
+  if (FLAG_generate_perf_events_symbols) {
+    UNIMPLEMENTED();
+  }
+#endif  // !PRODUCT
 }
 
 
 void OS::PrintErr(const char* format, ...) {
   va_list args;
   va_start(args, format);
   VFPrint(stderr, format, args);
   va_end(args);
 }
 
@@ skipping 17 matching lines @@
 }
 
 
 void OS::Exit(int code) {
   UNIMPLEMENTED();
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_FUCHSIA)