Avoid using RegExps in path to work around issue 19090.

pkg/path/lib/src/utils.dart

Index: pkg/path/lib/src/utils.dart
diff --git a/pkg/path/lib/src/utils.dart b/pkg/path/lib/src/utils.dart
new file mode 100644
index 0000000000000000000000000000000000000000..a59e122c42cf406812eddeb3741f1a6ee6ed7681
--- /dev/null
+++ b/pkg/path/lib/src/utils.dart
@@ -0,0 +1,13 @@
+// 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;
+
+/// Returns whether [char] is the code for an ASCII letter (uppercase or
+/// lowercase).
+bool isAlphabetic(int char) =>
+    (char >= 0x41 && char <= 0x5a) || (char >= 0x61 && char <= 0x7a);
+
+/// Returns whether [char] is the code for an ASCII digit.
+bool isNumeric(int char) => char >= 0x30 && char <= 0x39;