Serialize async marker on getters.

pkg/compiler/lib/src/serialization/element_serialization.dart

Index: pkg/compiler/lib/src/serialization/element_serialization.dart
diff --git a/pkg/compiler/lib/src/serialization/element_serialization.dart b/pkg/compiler/lib/src/serialization/element_serialization.dart
index d51c67ab15e4a0d184c1edbcc1e8b18c2f33f633..c50ca30b4717defaf056170e9117e837454f9fe4 100644
--- a/pkg/compiler/lib/src/serialization/element_serialization.dart
+++ b/pkg/compiler/lib/src/serialization/element_serialization.dart
@@ -544,6 +544,8 @@ class FunctionSerializer implements ElementSerializer {
     if (element.isFunction) {
       encoder.setBool(Key.IS_OPERATOR, element.isOperator);
       encoder.setEnum(Key.ASYNC_MARKER, element.asyncMarker);
+    } else if (element.isGetter) {
+      encoder.setEnum(Key.ASYNC_MARKER, element.asyncMarker);
     }
     SerializerUtil.serializeParentRelation(element, encoder);
     encoder.setBool(Key.IS_EXTERNAL, element.isExternal);