Make --trace-natives respect the isolate filter.

runtime/vm/native_entry.cc

Index: runtime/vm/native_entry.cc
diff --git a/runtime/vm/native_entry.cc b/runtime/vm/native_entry.cc
index 0420b4ac98ce7e77835f54ca04c9184404288393..ecd4f0c268e0113cf2747e5725379fadc495e6b9 100644
--- a/runtime/vm/native_entry.cc
+++ b/runtime/vm/native_entry.cc
@@ -271,7 +271,7 @@ void NativeEntry::LinkNativeCall(Dart_NativeArguments args) {
     const Function& func = Function::Handle(zone, code.function());
 
     if (FLAG_trace_natives) {
-      OS::Print("Resolving native target for %s\n", func.ToCString());
+      THR_Print("Resolving native target for %s\n", func.ToCString());
     }
 
     target_function =
@@ -320,7 +320,7 @@ void NativeEntry::LinkNativeCall(Dart_NativeArguments args) {
                                    patch_target_function, trampoline);
 
     if (FLAG_trace_natives) {
-      OS::Print("    -> %p (%s)\n", target_function,
+      THR_Print("    -> %p (%s)\n", target_function,
                 is_bootstrap_native ? "bootstrap" : "non-bootstrap");
     }
   }