Fix the initialization of PendingCallbacks in scheduled_test.

pkg/scheduled_test/lib/src/utils.dart

Index: pkg/scheduled_test/lib/src/utils.dart
diff --git a/pkg/scheduled_test/lib/src/utils.dart b/pkg/scheduled_test/lib/src/utils.dart
index 1f1485ab2b20c93757349702f74647cc70ce2c95..77de0fba4f5187c9f4c47887ffb780d19a7aa7ef 100644
--- a/pkg/scheduled_test/lib/src/utils.dart
+++ b/pkg/scheduled_test/lib/src/utils.dart
@@ -35,7 +35,6 @@ void chainToCompleter(Future future, Completer completer) {
 /// Prepends each line in [text] with [prefix]. If [firstPrefix] is passed, the
 /// first line is prefixed with that instead.
 String prefixLines(String text, {String prefix: '| ', String firstPrefix}) {
-  if (text == null) return '';
   var lines = text.split('\n');
   if (firstPrefix == null) {
     return lines.map((line) => '$prefix$line').join('\n');