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

runtime/bin/path_impl.dart

Index: runtime/bin/path_impl.dart
diff --git a/runtime/bin/path_impl.dart b/runtime/bin/path_impl.dart
index 9cdc3150f0d99f9ac2d2a9a25e7e659fe78b1f73..40b6aa7e095ececc5943a87dd5c7e11c1f6eb198 100644
--- a/runtime/bin/path_impl.dart
+++ b/runtime/bin/path_impl.dart
@@ -8,6 +8,8 @@ class _Path implements Path {
   const _Path(String source) : _path = source;
   _Path.fromNative(String source) : _path = _clean(source);
 
+  int hashCode() => _path.hashCode();
+
   static String _clean(String source) {
     switch (Platform.operatingSystem) {
       case 'windows':
@@ -179,7 +181,9 @@ class _Path implements Path {
   }
 
   Path append(String finalSegment) {
-    if (hasTrailingSeparator) {
+    if (isEmpty) {
+      return new Path(finalSegment);
+    } else if (hasTrailingSeparator) {
       return new Path('$_path$finalSegment');
     } else {
       return new Path('$_path/$finalSegment');