Update pub test with "Uncaught Error: " prefix on output.

sdk/lib/_internal/pub/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart

Index: sdk/lib/_internal/pub/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart
diff --git a/sdk/lib/_internal/pub/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart b/sdk/lib/_internal/pub/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart
index 643be22a2646e57a4eca5f0e375ab6e834957d2c..10e9c9118bb8a43dfa6cd56933874f1f9c680850 100644
--- a/sdk/lib/_internal/pub/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart
+++ b/sdk/lib/_internal/pub/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart
@@ -30,7 +30,7 @@ main() {
       createLockFile('myapp', pkg: ['barback']);
       var pub = startPubServe();
       pub.stderr.expect("'Unhandled exception:");
-      pub.stderr.expect(startsWith("Failure getting "));
+      pub.stderr.expect(startsWith("Uncaught Error: Failure getting "));
       pub.shouldExit(1);
     });
   });