Add GYP build rule for Try Dart!

dart/dart.gyp

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 {
   'targets': [
     {
       'target_name': 'most',
       'type': 'none',
       'dependencies': [
@@ skipping 166 matching lines @@
         ],
       ]
     },
     {
       'target_name': 'packages',
       'type': 'none',
       'dependencies': [
         'pkg/pkg.gyp:pkg_packages',
       ],
     },
+    {
+      'target_name': 'try',
+      'type': 'none',
+      'dependencies': [
+        'create_sdk',
+        'site/try/build_try.gyp:try_site',
+      ],
+    },
   ],
 }