Share code between the transformer isolate and pub proper.

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

Index: sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart b/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
index 82fa93418c8f46a3f26d0cc88855d57260fc84e7..a52a30e694901360a7b60c8a73f3807f9c3fd8bd 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
@@ -232,7 +232,7 @@ class BuildEnvironment {
     // infrastructure to share code with pub proper. We provide it only during
     // the initial transformer loading process.
     var dartPath = assetPath('dart');
-    var pubSources = listDir(dartPath).map((library) {
+    var pubSources = listDir(dartPath, recursive: true).map((library) {
       return new AssetId('\$pub',
           path.join('lib', path.relative(library, from: dartPath)));
     });