Add ServeLocalDirectory to the shell abstraction.

mojo/devtools/common/devtoolslib/shell.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 
 class Shell(object):
   """Represents an abstract Mojo shell."""
 
+  def ServeLocalDirectory(self, local_dir_path, port=0):
+    """Serves the content of the local (host) directory, making it available to
+    the shell under the url returned by the function.
+
+    The server will run on a separate thread until the program terminates. The
+    call returns immediately.
+
+    Args:
+      local_dir_path: path to the directory to be served
+      port: port at which the server will be available to the shell
+
+    Returns:
+      The url that the shell can use to access the content of |local_dir_path|.
+    """
+    raise NotImplementedError()
+
   def Run(self, arguments):
     """Runs the shell with given arguments until shell exits, passing the stdout
     mingled with stderr produced by the shell onto the stdout.
 
     Returns:
       Exit code retured by the shell or None if the exit code cannot be
       retrieved.
     """
     raise NotImplementedError()
 
   def RunAndGetOutput(self, arguments):
     """Runs the shell with given arguments until shell exits and returns the
     output.
 
     Args:
       arguments: list of arguments for the shell
 
     Returns:
       A tuple of (return_code, output). |return_code| is the exit code returned
       by the shell or None if the exit code cannot be retrieved. |output| is the
       stdout mingled with the stderr produced by the shell.
     """
     raise NotImplementedError()