Fuchsia: Fixes for Debug build

runtime/vm/os_fuchsia.cc

Index: runtime/vm/os_fuchsia.cc
diff --git a/runtime/vm/os_fuchsia.cc b/runtime/vm/os_fuchsia.cc
index 5272825c0370aa688a89ac512a0fcc84e00b4b38..9a4151f30a45dcab28f4a596a0a1f94688a85c72 100644
--- a/runtime/vm/os_fuchsia.cc
+++ b/runtime/vm/os_fuchsia.cc
@@ -15,6 +15,13 @@
 
 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";
 }
@@ -239,7 +246,11 @@ bool OS::StringToInt64(const char* str, int64_t* value) {
 
 
 void OS::RegisterCodeObservers() {
-  UNIMPLEMENTED();
+#ifndef PRODUCT
+  if (FLAG_generate_perf_events_symbols) {
+    UNIMPLEMENTED();
+  }
+#endif  // !PRODUCT
 }