Only run [declareOutputs] once for each asset/transformer pair.

pkg/barback/test/transformer/declaring_bad.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library barback.test.transformer.declaring_bad;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+import 'package:barback/src/utils.dart';
+
+import 'bad.dart';
+import 'mock.dart';
+
+/// A transformer that throws an exception when run, after generating the
+/// given outputs.
+class DeclaringBadTransformer extends MockTransformer
+    implements DeclaringTransformer {
+  final bool declareError;

[Bob Nystrom, 2014/04/08 18:23:44]

Document these.

[nweiz, 2014/04/08 23:42:33]

Done.

+  final bool applyError;
+  final String output;

[Bob Nystrom, 2014/04/08 18:23:44]

How about making this an AssetId and parsing it in the constructor?

[nweiz, 2014/04/08 23:42:33]

Done.

+
+  DeclaringBadTransformer(this.output, {bool declareError: true,
+          bool applyError: false})
+      : this.declareError = declareError,
+        this.applyError = applyError;
+
+  Future<bool> doIsPrimary(AssetId id) => new Future.value(true);
+
+  Future doApply(Transform transform) {
+    return newFuture(() {
+      var id = new AssetId.parse(output);
+      transform.addOutput(new Asset.fromString(id, "bad out"));
+      if (applyError) throw BadTransformer.ERROR;
+    });
+  }
+
+  Future declareOutputs(DeclaringTransform transform) {
+    return newFuture(() {
+      if (consumePrimary) transform.consumePrimary();
+      transform.declareOutput(new AssetId.parse(output));
+      if (declareError) throw BadTransformer.ERROR;
+    });
+  }
+}