Set dartAsyncMarker the same as asyncMarker unless explicitly set

pkg/kernel/lib/ast.dart

Index: pkg/kernel/lib/ast.dart
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index 8f05d4d8d178906ac103dc2aff8179f3c87936ab..2f833f394bf57e10189752187c97f071795162c4 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -1257,7 +1257,7 @@ class FunctionNode extends TreeNode {
       this.returnType: const DynamicType(),
       this.inferredReturnValue,
       this.asyncMarker: AsyncMarker.Sync,
-      this.dartAsyncMarker: AsyncMarker.Sync})
+      this.dartAsyncMarker})
       : this.positionalParameters =
             positionalParameters ?? <VariableDeclaration>[],
         this.requiredParameterCount =
@@ -1269,6 +1269,7 @@ class FunctionNode extends TreeNode {
     setParents(this.positionalParameters, this);
     setParents(this.namedParameters, this);
     body?.parent = this;
+    dartAsyncMarker ??= asyncMarker;
   }
 
   static DartType _getTypeOfVariable(VariableDeclaration node) => node.type;