Set dartAsyncMarker the same as asyncMarker unless explicitly set

pkg/kernel/lib/ast.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// -----------------------------------------------------------------------
 ///                           ERROR HANDLING
 /// -----------------------------------------------------------------------
 ///
 /// As a rule of thumb, errors that can be detected statically are handled by
 /// the frontend, typically by translating the erroneous code into a 'throw' or
@@ skipping 1239 matching lines @@
   Statement body;
 
   FunctionNode(this.body,
       {List<TypeParameter> typeParameters,
       List<VariableDeclaration> positionalParameters,
       List<VariableDeclaration> namedParameters,
       int requiredParameterCount,
       this.returnType: const DynamicType(),
       this.inferredReturnValue,
       this.asyncMarker: AsyncMarker.Sync,
-      this.dartAsyncMarker: AsyncMarker.Sync})
+      this.dartAsyncMarker})
       : this.positionalParameters =
             positionalParameters ?? <VariableDeclaration>[],
         this.requiredParameterCount =
             requiredParameterCount ?? positionalParameters?.length ?? 0,
         this.namedParameters = namedParameters ?? <VariableDeclaration>[],
         this.typeParameters = typeParameters ?? <TypeParameter>[] {
     assert(returnType != null);
     setParents(this.typeParameters, this);
     setParents(this.positionalParameters, this);
     setParents(this.namedParameters, this);
     body?.parent = this;
+    dartAsyncMarker ??= asyncMarker;
   }
 
   static DartType _getTypeOfVariable(VariableDeclaration node) => node.type;
 
   static NamedType _getNamedTypeOfVariable(VariableDeclaration node) {
     return new NamedType(node.name, node.type);
   }
 
   FunctionType get functionType {
     TreeNode parent = this.parent;
@@ skipping 2742 matching lines @@
 /// library has not been assigned a canonical name yet.
 ///
 /// Returns `null` if the library is `null`.
 CanonicalName getCanonicalNameOfLibrary(Library library) {
   if (library == null) return null;
   if (library.canonicalName == null) {
     throw '$library has no canonical name';
   }
   return library.canonicalName;
 }