Make sure that Dart depfile dependent is relative to build output

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")
 
 template("dartx") {
   bundle_prefix = target_name
   bundle = "$target_gen_dir/${bundle_prefix}.dartx"
   snapshot = "$target_gen_dir/${bundle_prefix}_snapshot.bin"
+  depfile_path = "${snapshot}.d"
 
   if (mojo_use_prebuilt_dart_snapshotter) {
     dart_snapshotter_path =
         rebase_path("mojo/public/tools:copy_dart_snapshotter", ".", mojo_root)
     dart_snapshotter_rule = "$dart_snapshotter_path($host_toolchain)"
   } else {
     dart_snapshotter_rule = dart_snapshotter_bin
   }
   dart_snapshotter_dir =
       get_label_info("$dart_snapshotter_rule", "root_out_dir")
   dart_snapshotter = "$dart_snapshotter_dir/dart_snapshotter"
 
   action("gen_${bundle_prefix}_snapshot") {
     main_dart = invoker.main_dart
 
+    depfile = depfile_path
+
     inputs = [
       dart_snapshotter,
     ]
     outputs = [
       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("$root_gen_dir/dart-pkg/packages"),
       "--snapshot",
       rebase_path(snapshot),
       "--depfile",
-      rebase_path(depfile),
+      rebase_path(depfile_path),
       "--build-output",
-      "${bundle_prefix}_snapshot.bin",
+      rebase_path(snapshot, root_build_dir),  # Relative to build directory
     ]
 
     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
     }
   }
 }