Support directories other than "web" in pub build.

sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart

Index: sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
index aa09244ed04baef220f855c6ed145c6783e178a1..136bfcd9fb03e7fd129387f4c3e1dfe027d72fbc 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -64,11 +64,15 @@ class Dart2JSTransformer extends Transformer {
   Dart2JSTransformer(PackageGraph graph, BarbackMode mode)
       : this.withSettings(graph, new BarbackSettings({}, mode));
 
-  /// Only ".dart" files within "web/" are processed.
+  /// Only ".dart" files within a buildable directory are processed.
   Future<bool> isPrimary(Asset asset) {
-    return new Future.value(
-        asset.id.extension == ".dart" &&
-        asset.id.path.startsWith("web/"));
+    if (asset.id.extension != ".dart") return new Future.value(false);
+
+    for (var dir in ["benchmark", "example", "test", "web"]) {
+      if (asset.id.path.startsWith("$dir/")) return new Future.value(true);
+    }
+
+    return new Future.value(false);
   }
 
   Future apply(Transform transform) {