Aggregate Transformer Build Error

[kzhdev]

I have an aggregate transformer which combines two JavaScript files into one. The transformer always cause an build error:

Build error:
Transforms {AlgoStoreMirror on adl_client|workerJs} all emitted asset adl_client|web/js/algo_store_mirror_worker.js.

Here is description of aggregate transformer on the web:

An aggregate transformer processes multiple assets in a single pass–for example, collaging multiple images into a single image.

Why it causes this build error?

Here is my transformer:

library adl_client.lib.algo_store_mirror_transformer;

import 'dart:async';
import 'package:barback/barback.dart';

class AlgoStoreMirrorTransformer extends AggregateTransformer {

  final List<String> includes = [
    'lib/js_src/vendor/q/q.js',
    'lib/js_src/services/algo-store/algo-store-mirror-worker.js.tpl'
  ];

  AlgoStoreMirrorTransformer.asPlugin();

  classifyPrimary(AssetId id) {
    if (includes.contains(id.path)) {
      return 'workerJs';
    }
    return null;
  }

  Future apply(AggregateTransform transform) async {
    var buffer = new StringBuffer();
    var assets = await transform.primaryInputs.toList();
    for (var asset in assets) {
      buffer.write(await asset.readAsString());
      buffer.write('\n');
    }
    var id = new AssetId(transform.package, 'web/js/algo_store_mirror_worker.js');
    transform.addOutput(new Asset.fromString(id, buffer.toString()));
  }
}