Add .fromPath constructors to File and Directory.

runtime/bin/file_impl.dart

Index: runtime/bin/file_impl.dart
diff --git a/runtime/bin/file_impl.dart b/runtime/bin/file_impl.dart
index 7900aef3586b6e35230033b015436d82b4db1c77..e1ff548fb669cfe7ba00be357490453da5ea5226 100644
--- a/runtime/bin/file_impl.dart
+++ b/runtime/bin/file_impl.dart
@@ -492,6 +492,9 @@ class _File extends _FileBase implements File {
   // Constructor for file.
   _File(String this._name);
 
+  // Constructor from Path for file.
+  _File.fromPath(Path path) : this(path.toNativePath());
+
   Future<bool> exists() {
     _ensureFileService();
     List request = new List(2);