Make Path implement (extend) Hashable, and make Path.append to an empty path work right.

runtime/bin/path.dart

Index: runtime/bin/path.dart
diff --git a/runtime/bin/path.dart b/runtime/bin/path.dart
index e1ef4fbd26a91dfe3ffa342acbe2382ed05a6626..80c711626de455d6cdf483c4f29cd1c56341edd9 100644
--- a/runtime/bin/path.dart
+++ b/runtime/bin/path.dart
@@ -9,7 +9,7 @@
  * useful path manipulations and queries.  Joining of paths and normalization
  * interpret '.' and '..' in the usual way.
  */
-interface Path default _Path {
+interface Path extends Hashable default _Path {
   /**
    * Creates a Path from the String [source].  [source] is used as-is, so if
    * the string does not consist of segments separated by forward slashes, the