Port sky_shell to Linux for testing

sky/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/ui.gni")
 import("//tools/grit/grit_rule.gni")
 
 group("sky") {
   testonly = true
 
@@ skipping 28 matching lines @@
     "//sky/sdk",
     "//third_party/dart-pkg",
   ]
 
   if (is_android) {
     deps += [
       "//sky/apk/demo",
       "//sky/services/intents",
     ]
   }
+
+  if (is_linux) {
+    deps += [ "//sky/shell" ]
+  }
 }