Add dep on dart_snapshotter_bin label in dartx rule

This requires that //build/module_args/dart.gni provide a label for
dart_snapshotter_bin and adds a dep in the dartx rule on the target. A
future patch will add a way to download a prebuilt binary.

R=zra@google.com, zra

Review URL: https://codereview.chromium.org/1294023004 .
diff --git a/build/module_args/dart.gni b/build/module_args/dart.gni
index ee6b038..e784261 100644
--- a/build/module_args/dart.gni
+++ b/build/module_args/dart.gni
@@ -4,3 +4,5 @@
 
 # This variable should point to the Dart SDK.
 dart_sdk_root = "//third_party/dart-sdk/dart-sdk"
+
+dart_snapshotter_bin = "//mojo/dart/dart_snapshotter($host_toolchain)"
diff --git a/mojo/public/dart/BUILD.gn b/mojo/public/dart/BUILD.gn
index b283bd0..a6fa7d0 100644
--- a/mojo/public/dart/BUILD.gn
+++ b/mojo/public/dart/BUILD.gn
@@ -1,3 +1,7 @@
+# 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.
+
 import("../mojo_sdk.gni")
 import("rules.gni")
 
diff --git a/mojo/public/dart/rules.gni b/mojo/public/dart/rules.gni
index f029041..7ef8c57 100644
--- a/mojo/public/dart/rules.gni
+++ b/mojo/public/dart/rules.gni
@@ -49,7 +49,9 @@
       rebase_path(snapshot),
     ]
 
-    deps = []
+    deps = [
+      dart_snapshotter_bin,
+    ]
     if (defined(invoker.deps)) {
       deps += invoker.deps
     }