Describe the bug
The graphql.schema.GraphQLSchema#newSchema(graphql.schema.GraphQLSchema) method does not copy the graphql.schema.GraphQLSchema.Builder#extensionDefinitions when generating a new builder from the existing GraphQLSchema. Is there a reason for this? Do I need to redefine the extensions when generating a new builder?

An easy fix would probably be:

return new Builder()
        ...
        .extensionDefinitions(existingSchema.getExtensionDefinitions())
        ...