Fix a reference error in pub's git source.

sdk/lib/_internal/pub/lib/src/source/git.dart

Index: sdk/lib/_internal/pub/lib/src/source/git.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source/git.dart b/sdk/lib/_internal/pub/lib/src/source/git.dart
index afec7bf0a70e15388184b967b7ae6c3611c9640a..41fef1f7310e78dee13fa96096d60947de79a182 100644
--- a/sdk/lib/_internal/pub/lib/src/source/git.dart
+++ b/sdk/lib/_internal/pub/lib/src/source/git.dart
@@ -141,7 +141,7 @@ class GitSource extends Source {
       // fetch" if possible to avoid networking time and errors. See if the
       // revision exists in the repo cache before updating it.
       return _revParse(id).catchError((error) {
-        if (error is! GitException) throw error;
+        if (error is! git.GitException) throw error;
         return _updateRepoCache(id).then((_) => _revParse(id));
       });
     });