Support loading transformer plugins from pub.

pkg/barback/test/package_graph/add_remove_transform_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.transform_test;
 
 import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
@@ skipping 156 matching lines @@
     // Ensure we're waiting on [rewrite.apply].
     schedule(pumpEventQueue);
 
     updateTransformers("app", []);
     rewrite.resumeApply();
     expectAsset("app|foo.blub", "foo");
     expectNoAsset("app|foo.blab");
     buildShouldSucceed();
   });
 
+  test("a transformer is added to an existing phase during isPrimary", () {
+    var rewrite = new RewriteTransformer("blub", "blab");
+    initGraph(["app|foo.blub", "app|bar.blib"], {"app": [[rewrite]]});
+
+    rewrite.pauseIsPrimary("app|foo.blub");
+    updateSources(["app|foo.blub", "app|bar.blib"]);
+    // Ensure we're waiting on [rewrite.isPrimary].
+    schedule(pumpEventQueue);
+
+    updateTransformers("app", [
+      [rewrite, new RewriteTransformer("blib", "blob")]
+    ]);
+    rewrite.resumeIsPrimary("app|foo.blub");
+    expectAsset("app|foo.blab", "foo.blab");
+    expectAsset("app|bar.blob", "bar.blob");
+    buildShouldSucceed();
+  });
+
   test("a new transformer can see pass-through assets", () {
     var rewrite = new RewriteTransformer("zip", "zap");
     initGraph(["app|foo.blub"], {"app": [[rewrite]]});
 
     updateSources(["app|foo.blub"]);
     buildShouldSucceed();
 
     updateTransformers("app", [
       [rewrite],
       [new RewriteTransformer("blub", "blab")]
@@ skipping 41 matching lines @@
 
     updateSources(["pkg1|foo.txt", "pkg2|foo.inc"]);
     expectAsset("pkg1|foo.out", "foo.inc.inc");
     buildShouldSucceed();
 
     updateTransformers("pkg2", [[rewrite]]);
     expectAsset("pkg1|foo.out", "foo.inc");
     buildShouldSucceed();
   });
 }