pkg/path: Add type annotations to top-level and static fields.

lib/path.dart

Index: lib/path.dart
diff --git a/lib/path.dart b/lib/path.dart
index cfbaf11fe078336c0e95b93db7fe0b33a870132c..af9efe54e849ded0200f1f45b469c32ac3c35bed 100644
--- a/lib/path.dart
+++ b/lib/path.dart
@@ -54,13 +54,13 @@ export 'src/path_exception.dart';
 export 'src/style.dart';
 
 /// A default context for manipulating POSIX paths.
-final posix = new Context(style: Style.posix);
+final Context posix = new Context(style: Style.posix);
 
 /// A default context for manipulating Windows paths.
-final windows = new Context(style: Style.windows);
+final Context windows = new Context(style: Style.windows);
 
 /// A default context for manipulating URLs.
-final url = new Context(style: Style.url);
+final Context url = new Context(style: Style.url);
 
 /// The system path context.
 ///