First stab at a dev server in pub using barback.

pkg/path/test/path_test.dart

Index: pkg/path/test/path_test.dart
diff --git a/pkg/path/test/path_test.dart b/pkg/path/test/path_test.dart
index ef3cf21ecd7f58e95ebc267cdd78369dd6dbfa70..d6c2f3493cfdf8fabfe3fdae104679b05c0073b1 100644
--- a/pkg/path/test/path_test.dart
+++ b/pkg/path/test/path_test.dart
@@ -34,4 +34,19 @@ main() {
       expect(builder.style, path.Style.windows);
     });
   });
+
+  test('posix is a default Builder for the POSIX style', () {
+    expect(path.posix.style, path.Style.posix);
+    expect(path.posix.root, ".");
+  });
+
+  test('windows is a default Builder for the Windows style', () {
+    expect(path.windows.style, path.Style.windows);
+    expect(path.windows.root, ".");
+  });
+
+  test('url is a default Builder for the URL style', () {
+    expect(path.url.style, path.Style.url);
+    expect(path.url.root, ".");
+  });
 }