Regenerate .mojo file if any dependent .dart sources change

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.
 # The entrypoint is the dart_pkg rule.
 
 import("../mojo.gni")
 import("//build/module_args/mojo.gni")
 import("//build/module_args/dart.gni")
@@ skipping 24 matching lines @@
       snapshot,
     ]
 
     if (defined(invoker.sources)) {
       sources = invoker.sources
     }
 
     script =
         rebase_path("mojo/public/tools/dart_snapshotter.py", ".", mojo_sdk_root)
 
+    depfile = "$target_gen_dir/${bundle_prefix}_snapshot.bin.d"
+
     args = [
       rebase_path(dart_snapshotter),
       rebase_path(main_dart),
       "--package-root",
       rebase_path("$root_gen_dir/dart-pkg/packages"),
       "--snapshot",
       rebase_path(snapshot),
+      "--depfile",
+      rebase_path(depfile),
+      "--build-output",
+      "${bundle_prefix}_snapshot.bin",
     ]
 
     deps = [
       dart_snapshotter_rule,
     ]
     if (defined(invoker.deps)) {
       deps += invoker.deps
     }
   }
 
@@ skipping 380 matching lines @@
   group(target_name) {
     deps = []
     foreach(package_dir, packages) {
       deps += [ ":$package_dir" ]
     }
     if (defined(invoker.deps)) {
       deps += invoker.deps
     }
   }
 }