Convert shelf to use the string_scanner package.

pkg/shelf/lib/src/string_scanner.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 shelf.string_scanner;
-
-/// A class that scans through a string using [Pattern]s.
-class StringScanner {
-  /// The string being scanned through.
-  final String string;
-
-  /// The current position of the scanner in the string, in characters.
-  int get position => _position;
-  set position(int position) {
-    if (position < 0 || position > string.length) {
-      throw new ArgumentError("Invalid position $position");
-    }
-
-    _position = position;
-  }
-  int _position = 0;
-
-  /// The data about the previous match made by the scanner.
-  ///
-  /// If the last match failed, this will be `null`.
-  Match get lastMatch => _lastMatch;
-  Match _lastMatch;
-
-  /// The portion of the string that hasn't yet been scanned.
-  String get rest => string.substring(position);
-
-  /// Whether the scanner has completely consumed [string].
-  bool get isDone => position == string.length;
-
-  /// Creates a new [StringScanner] that starts scanning from [position].
-  ///
-  /// [position] defaults to 0, the beginning of the string.
-  StringScanner(this.string, {int position}) {
-    if (position != null) this.position = position;
-  }
-
-  /// If [pattern] matches at the current position of the string, scans forward
-  /// until the end of the match.
-  ///
-  /// Returns whether or not [pattern] matched.
-  bool scan(Pattern pattern) {
-    var success = matches(pattern);
-    if (success) _position = _lastMatch.end;
-    return success;
-  }
-
-  /// If [pattern] matches at the current position of the string, scans forward
-  /// until the end of the match.
-  ///
-  /// If [pattern] did not match, throws a [FormatException] with [message].
-  void expect(Pattern pattern, String message) {
-    if (scan(pattern)) return;
-    throw new FormatException(message);
-  }
-
-  /// Returns whether or not [pattern] matches at the current position of the
-  /// string.
-  ///
-  /// This doesn't move the scan pointer forward.
-  bool matches(Pattern pattern) {
-    _lastMatch = pattern.matchAsPrefix(string, position);
-    return _lastMatch != null;
-  }
-}