Change package scheduled_test to throw TestFailures rather than strings.

pkg/scheduled_test/lib/scheduled_process.dart

Index: pkg/scheduled_test/lib/scheduled_process.dart
diff --git a/pkg/scheduled_test/lib/scheduled_process.dart b/pkg/scheduled_test/lib/scheduled_process.dart
index 4768c684fefe3ef83f668fe41e432224ec48a100..4b2830979e5ba63c3983b31207efce9d11352a9c 100644
--- a/pkg/scheduled_test/lib/scheduled_process.dart
+++ b/pkg/scheduled_test/lib/scheduled_process.dart
@@ -183,8 +183,8 @@ class ScheduledProcess {
         exitCodeCompleter.complete(exitCode);
 
         if (!_endExpected) {
-          throw "Process '$description' ended earlier than scheduled "
-            "with exit code $exitCode.";
+          fail("Process '$description' ended earlier than scheduled "
+               "with exit code $exitCode.");
         }
       }), "waiting to reach shouldExit() or kill() for process "
           "'$description'");