Convert shelf to use the string_scanner package.

pkg/string_scanner/test/utils.dart

+// Copyright (c) 2014, 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.
+
+library string_scanner.test.utils;
+
+import 'package:unittest/unittest.dart';
+
+/// Returns a matcher that asserts that a closure throws a [FormatException]
+/// with the given [message].
+Matcher throwsFormattedError(String message) {
+  return throwsA(predicate((error) {
+    expect(error, isFormatException);
+    expect(error.message, equals(message));
+    return true;
+  }));
+}