Support both new and old style package layouts.

utils/pub/sdk_source.dart

Index: utils/pub/sdk_source.dart
diff --git a/utils/pub/sdk_source.dart b/utils/pub/sdk_source.dart
index 5984555fe3bb300bfcb0fd6d313597e4fc2f0218..9086128f553cf74ca0c792b44d37ae8281bc17f0 100644
--- a/utils/pub/sdk_source.dart
+++ b/utils/pub/sdk_source.dart
@@ -58,10 +58,12 @@ class SdkSource extends Source {
         sourcePath = join(rootDir, "lib", id.description);
         return exists(sourcePath).chain((found) {
           if (!found) return new Future<bool>.immediate(false);
-          return createSymlink(sourcePath, destPath).transform((_) => true);
+          return createPackageSymlink(sourcePath, destPath).transform(
+              (_) => true);
         });
       }
-      return createSymlink(sourcePath, destPath).transform((_) => true);
+
+      return createPackageSymlink(sourcePath, destPath).transform((_) => true);
     });
   }
 }