Update the gen command to not fail on generator flags.

mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart

Index: mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart
diff --git a/mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart b/mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart
index f14216eaf6ef9176e8e008d27e62be7ce081600c..362d8490294e7dd14426edd15bb48d7bcb3052ff 100644
--- a/mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart
+++ b/mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart
@@ -765,7 +765,7 @@ mojom_types.RuntimeTypeInfo  _initRuntimeTypeInfo() {
   // serializedRuntimeTypeInfo contains the bytes of the Mojo serialization of
   // a mojom_types.RuntimeTypeInfo struct describing the Mojom types in this
   // file. The string contains the base64 encoding of the gzip-compressed bytes.
-  var serializedRuntimeTypeInfo = "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";
+  var serializedRuntimeTypeInfo = "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";
 
   // Deserialize RuntimeTypeInfo
   var bytes = BASE64.decode(serializedRuntimeTypeInfo);