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

pkg/barback/test/transformer/declare_assets.dart

Index: pkg/barback/test/transformer/declare_assets.dart
diff --git a/pkg/barback/test/transformer/declare_assets.dart b/pkg/barback/test/transformer/declare_assets.dart
new file mode 100644
index 0000000000000000000000000000000000000000..9a14525f7edcc3114463386d2840542f9efa9947
--- /dev/null
+++ b/pkg/barback/test/transformer/declare_assets.dart
@@ -0,0 +1,44 @@
+// 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.declare_asset;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+import 'package:barback/src/utils.dart';
+
+import 'mock.dart';
+
+/// A transformer that declares some outputs and emits others.
+class DeclareAssetsTransformer extends MockTransformer
+    implements DeclaringTransformer {
+  /// The assets that the transformer declares that it will emit.
+  final List<AssetId> declared;
+
+  /// The assets that the transformer actually emits.
+  ///
+  /// These assets' contents will be identical to their ids.
+  final List<AssetId> emitted;
+
+  DeclareAssetsTransformer(Iterable<String> declared, Iterable<String> emitted)
+      : this.declared = declared.map((id) => new AssetId.parse(id)).toList(),
+        this.emitted = emitted.map((id) => new AssetId.parse(id)).toList();
+
+  Future<bool> doIsPrimary(AssetId id) => new Future.value(true);
+
+  Future doApply(Transform transform) {
+    return newFuture(() {
+      for (var id in emitted) {
+        transform.addOutput(new Asset.fromString(id, id.toString()));
+      }
+    });
+  }
+
+  Future declareOutputs(DeclaringTransform transform) {
+    return newFuture(() {
+      declared.forEach(transform.declareOutput);
+    });
+  }
+}