Add an AuthenticationChallenge class.

lib/src/utils.dart

Index: lib/src/utils.dart
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4dcee1988000e1a0d77be60a35765881625bd753
--- /dev/null
+++ b/lib/src/utils.dart
@@ -0,0 +1,23 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:source_span/source_span.dart';
+
+/// Runs [body] and wraps any format exceptions it produces.
+///
+/// [name] should describe the type of thing being parsed, and [value] should be
+/// its actual value.
+wrapFormatException(String name, String value, body()) {
+  try {
+    return body();
+  } on SourceSpanFormatException catch (error) {
+    throw new SourceSpanFormatException(
+        'Invalid $name: ${error.message}', error.span, error.source);
+  } on FormatException catch (error) {
+    throw new FormatException(
+        'Invalid $name "$value": ${error.message}',
+        error.source,
+        error.offset);
+  }
+}