Lazily throw pubspec parsing exceptions.

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

Index: sdk/lib/_internal/pub/lib/src/barback.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback.dart b/sdk/lib/_internal/pub/lib/src/barback.dart
index 3c14a16fea84963ddfcc8f946e8ac667cfe004ab..2d0b6006180c62ac035aa6c2760f9983ee1becf2 100644
--- a/sdk/lib/_internal/pub/lib/src/barback.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback.dart
@@ -33,9 +33,8 @@ class TransformerId {
     if (configuration == null) return;
     for (var reserved in ['include', 'exclude']) {
       if (!configuration.containsKey(reserved)) continue;
-      throw new FormatException('Configuration for transformer '
-          '${idToLibraryIdentifier(asset)} may not include reserved key '
-          '"$reserved".');
+      throw new FormatException('Transformer configuration may not include '
+          'reserved key "$reserved".');
     }
   }