Add missing methods to 2.7/posixpath.pyi. (#622)

stdlib/2.7/posixpath.pyi

@@ -2,7 +2,7 @@
 #
 # NOTE: This dynamically typed stub was automatically generated by stubgen.
 
-from typing import Any
+from typing import Any, AnyStr, List
 from genericpath import *
 
 curdir = ... # type: Any
@@ -39,12 +39,12 @@ supports_unicode_filenames = ... # type: Any
 
 def relpath(path, start=...): ...
 
-# Names in __all__ with no definition:
-#   commonprefix
-#   exists
-#   getatime
-#   getctime
-#   getmtime
-#   getsize
-#   isdir
-#   isfile
+# posixpath imports these from genericpath.py:
+def commonprefix(list: List[AnyStr]) -> AnyStr: ...
+def exists(path: unicode) -> bool: ...
+def getatime(path: unicode) -> float: ...
+def getmtime(path: unicode) -> float: ...
+def getctime(path: unicode) -> float: ...
+def getsize(path: unicode) -> int: ...
+def isfile(path: unicode) -> bool: ...
+def isdir(path: unicode) -> bool: ...