Use backtracking when solving dependency constraints.

utils/tests/pub/test_pub.dart

Index: utils/tests/pub/test_pub.dart
diff --git a/utils/tests/pub/test_pub.dart b/utils/tests/pub/test_pub.dart
index bf3722b422c35ca267669eea6db711dea6f4a9cc..13960e5b93e1e535f12f29cb3b64522d79540ed0 100644
--- a/utils/tests/pub/test_pub.dart
+++ b/utils/tests/pub/test_pub.dart
@@ -22,6 +22,7 @@ import '../../../pkg/pathos/lib/path.dart' as path;
 import '../../../pkg/scheduled_test/lib/scheduled_process.dart';
 import '../../../pkg/scheduled_test/lib/scheduled_server.dart';
 import '../../../pkg/scheduled_test/lib/scheduled_test.dart';
+import '../../../pkg/unittest/lib/unittest.dart' as unittest;
 import '../../../pkg/yaml/lib/yaml.dart';
 
 import '../../lib/file_system.dart' as fs;
@@ -47,7 +48,7 @@ import 'descriptor.dart' as d;
 initConfig() {
   // If we aren't running on the bots, use the human-friendly config.
   if (new Options().arguments.contains('--human')) {
-    configure(new CommandLineConfiguration());
+    unittest.unittestConfiguration = new CommandLineConfiguration();
   }
 }