Only read PC marker when we are executing Dart code

runtime/vm/profiler.cc

Index: runtime/vm/profiler.cc
diff --git a/runtime/vm/profiler.cc b/runtime/vm/profiler.cc
index 83e2235f51636015c73abf847d78940388e50d31..444a5794af298c96df4cedc4fce1e47b828e44cb 100644
--- a/runtime/vm/profiler.cc
+++ b/runtime/vm/profiler.cc
@@ -1681,6 +1681,7 @@ static void SetPCMarkerIfSafe(Sample* sample) {
     // the pc marker, because we are reading it from a different thread on
     // Windows. The marker is below fp and the previous page may be a guard
     // page.
+    COMPILE_ASSERT(kPcMarkerSlotFromFp < 0);
     const intptr_t kPageMask = VirtualMemory::PageSize() - 1;
     if ((sample->fp() & kPageMask) == 0) {
       return;
@@ -2048,12 +2049,10 @@ void Profiler::RecordSampleInterruptCallback(
   sample->set_user_tag(isolate->user_tag());
   sample->set_sp(state.sp);
   sample->set_fp(state.fp);
-#if !(defined(TARGET_OS_WINDOWS) && defined(TARGET_ARCH_X64))
-  // It is never safe to read other thread's stack unless on Win64
-  // other thread is inside Dart code.
-  SetPCMarkerIfSafe(sample);
-#endif
-
+  if (isolate->vm_tag() == VMTag::kDartTagId) {
+    // We should only read the PC marker when executing Dart code.
+    SetPCMarkerIfSafe(sample);
+  }
   // Walk the call stack.
   if (FLAG_profile_vm) {
     // Always walk the native stack collecting both native and Dart frames.