Change package scheduled_test to throw TestFailures rather than strings.

pkg/scheduled_test/lib/src/descriptor/nothing_descriptor.dart

Index: pkg/scheduled_test/lib/src/descriptor/nothing_descriptor.dart
diff --git a/pkg/scheduled_test/lib/src/descriptor/nothing_descriptor.dart b/pkg/scheduled_test/lib/src/descriptor/nothing_descriptor.dart
index 8a80be259c092762562e9c2cb5f1825d7d72c7f7..590a1ce140afe3c32c750ded8664fbbdae2c5fe3 100644
--- a/pkg/scheduled_test/lib/src/descriptor/nothing_descriptor.dart
+++ b/pkg/scheduled_test/lib/src/descriptor/nothing_descriptor.dart
@@ -27,10 +27,9 @@ class NothingDescriptor extends Descriptor {
     if (parent == null) parent = defaultRoot;
     var fullPath = path.join(parent, name);
     if (new File(fullPath).existsSync()) {
-      throw "Expected nothing to exist at '$fullPath', but found a file.";
+      fail("Expected nothing to exist at '$fullPath', but found a file.");
     } else if (new Directory(fullPath).existsSync()) {
-      throw "Expected nothing to exist at '$fullPath', but found a "
-          "directory.";
+      fail("Expected nothing to exist at '$fullPath', but found a directory.");
     } else {
       return;
     }