Track async causal stack traces

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index ee95896ceb1d435266f6497ed811daafe98321a5..71cef9f50143a18d062259a2828ec31c1914691c 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -5114,6 +5114,11 @@ RawString* Api::GetEnvironmentValue(Thread* thread, const String& name) {
     if (Symbols::DartIsVM().Equals(name)) {
       return Symbols::True().raw();
     }
+    if (FLAG_causal_async_stacks) {
+      if (Symbols::DartDeveloperCausalAsyncStacks().Equals(name)) {
+        return Symbols::True().raw();
+      }
+    }
   }
   return result.raw();
 }