First pass at build dependency graph for barback.

sdk/lib/_internal/pub/lib/src/path_source.dart

Index: sdk/lib/_internal/pub/lib/src/path_source.dart
diff --git a/sdk/lib/_internal/pub/lib/src/path_source.dart b/sdk/lib/_internal/pub/lib/src/path_source.dart
index 597205051790cc882398ee0917833e519f289f42..2d3cbf4c96c5ab2b5f0651aa41feb5b642739afb 100644
--- a/sdk/lib/_internal/pub/lib/src/path_source.dart
+++ b/sdk/lib/_internal/pub/lib/src/path_source.dart
@@ -52,6 +52,13 @@ class PathSource extends Source {
     });
   }
 
+  Future<String> getDirectory(PackageId id) {
+    return new Future(() {
+      _validatePath(id.name, id.description);
+      return id.description["path"];
+    });
+  }
+
   /// Parses a path dependency. This takes in a path string and returns a map.
   /// The "path" key will be the original path but resolved relative to the
   /// containing path. The "relative" key will be `true` if the original path