Files services library: Add an "output stream" file impl helper class.

mojo/tools/data/apptests

 # This file contains a list of Mojo apptests. For description of the file
 # format, see `mojo_test` in devtools.
 
 tests = [
   {
     "test": "mojo:asset_bundle_apptests",
   },
   {
     "test": "mojo:authenticating_url_loader_interceptor_apptests",
   },
@@ skipping 12 matching lines @@
     "test": "mojo:example_apptests",
     "name": "mojo:example_apptests (multiprocess)",
     # ExampleApplicationTest.CheckCommandLineArg checks --example_apptest_arg.
     "test-args": ["--example_apptest_arg"],
     "shell-args": ["--enable-multiprocess"],
   },
   {
     "test": "mojo:files_apptests",
   },
   {
+    "test": "mojo:files_impl_apptests",
+  },
+  {
     "test": "mojo:http_server_apptests",
   },
   {
     "test": "mojo:mojio_apptests",
   },
   {
     "test": "mojo:moterm_apptests",
     "shell-args": ["--args-for=mojo:native_viewport_service --use-headless-config --use-osmesa"],
   },
   {
@@ skipping 78 matching lines @@
       "test": "mojo:dart_apptests",
       "type": "dart",
     },
     {
       # https://github.com/domokit/mojo/issues/61
       # Sometime the shell get a trucated application when exposed through the
       # http server.
       "test": "mojo:shell_apptests",
     },
   ]