Add dep on dart_snapshotter_bin label in dartx rule

mojo/public/dart/rules.gni

 # 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.
 
 # This file has rules for making Dart packages and Dart-based Mojo applications.
 #
 # Entrypoint rules are:
 # - dart_pkg
 # - dartzip_package
 # - dartzip_packaged_application
@@ skipping 31 matching lines @@
 
     args = [
       rebase_path(dart_snapshotter),
       rebase_path(main_dart),
       "--package-root",
       rebase_path("$root_gen_dir/dart-pkg/packages"),
       "--snapshot",
       rebase_path(snapshot),
     ]
 
-    deps = []
+    deps = [
+      dart_snapshotter_bin,

[jamesr, 2015/08/17 21:05:25]

If this variable isn't set, this will blow up at GN time.  There isn't a better
option at this point, however.

+    ]
     if (defined(invoker.deps)) {
       deps += invoker.deps
     }
   }
 
   action("gen_${bundle_prefix}_bundle") {
     sources = [
       rebase_path("mojo/public/tools/dartx.py", ".", mojo_sdk_root),
       snapshot,
     ]
@@ skipping 533 matching lines @@
 
     rebase_input = rebase_path(input, root_build_dir)
     rebase_output = rebase_path(output, root_build_dir)
     args = [
       "--input=$rebase_input",
       "--output=$rebase_output",
       "--line=$line",
     ]
   }
 }