Gracefully handle pubspecs with dependencies using unknown sources.

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 2b66e3b6b5b228d3cf3335b52205d273dcd5238e..597205051790cc882398ee0917833e519f289f42 100644
--- a/sdk/lib/_internal/pub/lib/src/path_source.dart
+++ b/sdk/lib/_internal/pub/lib/src/path_source.dart
@@ -23,7 +23,7 @@ class PathSource extends Source {
   final name = 'path';
   final shouldCache = false;
 
-  Future<Pubspec> describe(PackageId id) {
+  Future<Pubspec> describeUncached(PackageId id) {
     return new Future.sync(() {
       _validatePath(id.name, id.description);
       return new Pubspec.load(id.name, id.description["path"],