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..df9826f5bf9c2a6c978d80d0fb9dbbaa08e822af 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 +1269,9 @@ class FunctionNode extends TreeNode {
     setParents(this.positionalParameters, this);
     setParents(this.namedParameters, this);
     body?.parent = this;
+    if (dartAsyncMarker == null) {

[Kevin Millikin (Google), 2017/03/02 10:10:26]

dartAsyncMarker ??= asyncMarker;

+      dartAsyncMarker = asyncMarker;
+    }
   }
 
   static DartType _getTypeOfVariable(VariableDeclaration node) => node.type;