Add 'Convert Method to Getter' proposal only if it is an available refactoring.

pkg/analysis_server/test/services/refactoring/convert_method_to_getter_test.dart

Index: pkg/analysis_server/test/services/refactoring/convert_method_to_getter_test.dart
diff --git a/pkg/analysis_server/test/services/refactoring/convert_method_to_getter_test.dart b/pkg/analysis_server/test/services/refactoring/convert_method_to_getter_test.dart
index 32780d8ed9763fb040a6163fa9dd09e72891623f..eea53857c2c38485802e07c00e21539688fb08e4 100644
--- a/pkg/analysis_server/test/services/refactoring/convert_method_to_getter_test.dart
+++ b/pkg/analysis_server/test/services/refactoring/convert_method_to_getter_test.dart
@@ -148,6 +148,15 @@ main() {
         'Only methods without parameters can be converted to getters.');
   }
 
+  test_checkInitialConditions_returnTypeVoid() {
+    indexTestUnit('''
+void test() {}
+''');
+    _createRefactoring('test');
+    // check conditions
+    _assertInitialConditions_fatal('Cannot convert function returning void.');
+  }
+
   test_checkInitialConditions_localFunction() {
     indexTestUnit('''
 main() {