Avoid using RegExps in path to work around issue 19090.

pkg/path/lib/src/characters.dart

Index: pkg/path/lib/src/characters.dart
diff --git a/pkg/path/lib/src/characters.dart b/pkg/path/lib/src/characters.dart
new file mode 100644
index 0000000000000000000000000000000000000000..ff196a6722596dec232eba9976a5b151999e42d2
--- /dev/null
+++ b/pkg/path/lib/src/characters.dart
@@ -0,0 +1,19 @@
+// 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.
+
+/// This library contains character-code definitions.
+library path.characters;
+
+const PLUS = 0x2b;
+const MINUS = 0x2d;
+const PERIOD = 0x2e;
+const SLASH = 0x2f;
+const ZERO = 0x30;
+const NINE = 0x39;
+const COLON = 0x3a;
+const UPPER_A = 0x41;
+const UPPER_Z = 0x5a;
+const LOWER_A = 0x61;
+const LOWER_Z = 0x7a;
+const BACKSLASH = 0x5c;