Add BaseTransform.consumePrimary to barback.

pkg/barback/test/transformer/emit_nothing.dart

+// Copyright (c) 2014, 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.emit_nothing;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+import 'mock.dart';
+
+/// A transformer that emits no assets.
+class EmitNothingTransformer extends MockTransformer {
+  final String extension;
+
+  EmitNothingTransformer(this.extension);
+
+  Future<bool> doIsPrimary(Asset asset) =>
+    new Future.value(asset.id.extension == ".$extension");
+
+  Future doApply(Transform transform) => new Future.value();
+
+  String toString() => "$extension->nothing";
+}