Fix build: _addFix_addAsync_asyncFor() does not return.

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index a400a0cd5f0fffd706a15639ab8df5bfab4cf529..e223f815dab08891a41125cfcca482448ea17c56 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -150,7 +150,7 @@ class FixProcessor {
       _addFix_replaceWithConstInstanceCreation();
     }
     if (errorCode == CompileTimeErrorCode.ASYNC_FOR_IN_WRONG_CONTEXT) {
-      bool isAsync = _addFix_addAsync_asyncFor();
+      _addFix_addAsync_asyncFor();
     }
     if (errorCode == CompileTimeErrorCode.INVALID_ANNOTATION) {
       if (node is Annotation) {