Support loading transformer plugins from pub.

pkg/barback/test/package_graph/add_remove_transform_test.dart

Index: pkg/barback/test/package_graph/add_remove_transform_test.dart
diff --git a/pkg/barback/test/package_graph/add_remove_transform_test.dart b/pkg/barback/test/package_graph/add_remove_transform_test.dart
index bd68f68b2c3acea7c8eef3538fcd6ff32fb5a423..932dc0fde19ef446f3be1e8630caf8d6521cb284 100644
--- a/pkg/barback/test/package_graph/add_remove_transform_test.dart
+++ b/pkg/barback/test/package_graph/add_remove_transform_test.dart
@@ -174,6 +174,24 @@ main() {
     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]]});