Unwrap AsyncErrors to find real error.

utils/pub/utils.dart

Index: utils/pub/utils.dart
diff --git a/utils/pub/utils.dart b/utils/pub/utils.dart
index b922d845d4013de9e3132acb6033f3e2e5fa86b4..544a3ce0bef5c1c9a9d99dc98154aa04694a91d3 100644
--- a/utils/pub/utils.dart
+++ b/utils/pub/utils.dart
@@ -183,3 +183,14 @@ void mapAddAll(Map destination, Map source) =>
 /// replacing `+` with ` `.
 String urlDecode(String encoded) =>
   decodeUriComponent(encoded.replaceAll("+", " "));
+
+// TODO(rnystrom): Remove this when #7781 is fixed.
+/// When an error is rethrown in an async callback, you can end up with nested
+/// AsyncErrors. This unwraps them to find the real originating error.
+getRealError(error) {
+  while (error is AsyncError) {
+    error = error.error;
+  }
+
+  return error;
+}