add parser parseAsync flag

pkg/analyzer/test/src/task/options_test.dart

Index: pkg/analyzer/test/src/task/options_test.dart
diff --git a/pkg/analyzer/test/src/task/options_test.dart b/pkg/analyzer/test/src/task/options_test.dart
index c3f670b4e9247465cb6532489b5de56ad45203d4..ebdca704569074a62dbe46923a3cc7f3e78bb321 100644
--- a/pkg/analyzer/test/src/task/options_test.dart
+++ b/pkg/analyzer/test/src/task/options_test.dart
@@ -50,6 +50,23 @@ analyzer:
     expect(analysisOptions.strongMode, false);
   }
 
+  test_configure_enableAsync() {
+    configureContext('''
+analyzer:
+  language:
+''');
+    expect(analysisOptions.enableAsync, true);
+  }
+
+  test_configure_enableAsync_false() {
+    configureContext('''
+analyzer:
+  language:
+    enableAsync: false
+''');
+    expect(analysisOptions.enableAsync, false);
+  }
+
   test_configure_enableGenericMethods() {
     expect(analysisOptions.enableGenericMethods, false);
     configureContext('''