Fix a bug when loading secondary assets in barback.

pkg/barback/test/package_graph/group_test.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.package_graph.group_test;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:scheduled_test/scheduled_test.dart';
@@ skipping 431 matching lines @@
     initGraph(["app|foo.a", "app|foo.x"], {"app": [[
       new TransformerGroup([
         [new RewriteTransformer("a", "b")],
         [new RewriteTransformer("b", "c")]
       ]),
       new RewriteTransformer("x", "c")
     ]]});
     updateSources(["app|foo.a", "app|foo.x"]);
     buildShouldFail([isAssetCollisionException("app|foo.c")]);
   });
+
+  // Regression test for issue 18872.
+  test("a multi-phase group's outputs should be visible as secondary inputs "
+      "for a following group", () {
+    initGraph({
+      "app|foo.txt": "bar.c",
+      "app|bar.a": "bar"
+    }, {"app": [
+      [new TransformerGroup([
+        [new RewriteTransformer("a", "b")],
+        [new RewriteTransformer("b", "c")]
+      ])],
+      [new TransformerGroup([
+        [new ManyToOneTransformer("txt")]
+      ])]
+    ]});
+
+    updateSources(["app|foo.txt", "app|bar.a"]);
+    expectAsset("app|foo.out", "bar.b.c");
+    buildShouldSucceed();
+  });
 }