More test tweaks:

- Fix a test description (cl/324657206)
- Add return type to a helper

Author: osa1

protoc_plugin/test/omit_field_names_test.dart

@@ -10,7 +10,7 @@ import '../out/protos/google/protobuf/unittest.pb.dart';
 String constant() => 'SHOULD_BE_PRESENT';
 
 Future<void> main() async {
-  test('enum name available depending on environment', () {
+  test('field name available depending on environment', () {
     var proto = TestAllTypes()..optionalForeignMessage = ForeignMessage();
 
     expect(

protoc_plugin/test/protoc_options_test.dart

@@ -25,7 +25,7 @@ void main() {
   });
 
   test('testInvalidGeneratorOptions', () {
-    checkInvalid(String parameter) {
+    void checkInvalid(String parameter) {
       var request = CodeGeneratorRequest();
       request.parameter = parameter;
       var response = CodeGeneratorResponse();