Avoid using RegExps in path to work around issue 19090.

pkg/path/lib/src/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 path.utils;
+
+import 'characters.dart' as chars;
+
+/// Returns whether [char] is the code for an ASCII letter (uppercase or
+/// lowercase).
+bool isAlphabetic(int char) =>
+    (char >= chars.UPPER_A && char <= chars.UPPER_Z) ||
+    (char >= chars.LOWER_A && char <= chars.LOWER_Z);
+
+/// Returns whether [char] is the code for an ASCII digit.
+bool isNumeric(int char) => char >= chars.ZERO && char <= chars.NINE;