Correctly handle abstract non-sync methods.

pkg/front_end/lib/src/fasta/source/diet_listener.dart

Index: pkg/front_end/lib/src/fasta/source/diet_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/source/diet_listener.dart b/pkg/front_end/lib/src/fasta/source/diet_listener.dart
index edd4c1258729e929b1a4453a8e01dbe097193f14..6eb987e3fb252736ec2c8180cfe447a9a0471fe5 100644
--- a/pkg/front_end/lib/src/fasta/source/diet_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/diet_listener.dart
@@ -489,9 +489,9 @@ class DietListener extends StackListener {
       listener.prepareInitializers();
       token = parser.parseInitializersOpt(token);
       token = parser.parseAsyncModifier(token);
-      AsyncMarker asyncModifier = getAsyncMarker(listener);
+      AsyncMarker asyncModifier = getAsyncMarker(listener) ?? AsyncMarker.Sync;
       bool isExpression = false;
-      bool allowAbstract = true;
+      bool allowAbstract = asyncModifier == AsyncMarker.Sync;
       parser.parseFunctionBody(token, isExpression, allowAbstract);
       var body = listener.pop();
       listener.checkEmpty(token.charOffset);