addConstantImportsTo method
Adds dependencies of generateConstants to imports.
For each .pbjson.dart file that the generated code needs to import, add its generator.
Implementation
void addConstantImportsTo(Set<FileGenerator> imports) {
  checkResolved();
  for (final m in _messageGenerators) {
    m.addConstantImportsTo(imports);
  }
  for (final x in _extensionGenerators) {
    x.addConstantImportsTo(imports);
  }
}