Allow caller of application_snapshot to override .packages

utils/application_snapshot.gni

 # Copyright (c) 2016, 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.
 
 import("../build/compiled_action.gni")
 
 _dart_root = get_path_info("..", "abspath")
 
 declare_args() {
   # Default to building app-jit snapshots. The simulator and cross builds
@@ skipping 14 matching lines @@
     name = invoker.name
   }
   extra_deps = []
   if (defined(invoker.deps)) {
     extra_deps += invoker.deps
   }
   extra_inputs = [ main_dart ]
   if (defined(invoker.inputs)) {
     extra_inputs += invoker.inputs
   }
+  if (defined(invoker.dot_packages)) {
+    dot_packages = invoker.dot_packages
+  } else {
+    dot_packages = rebase_path("$_dart_root/.packages")
+  }
   compiled_action(target_name) {
     tool = "$_dart_root/runtime/bin:dart"
     deps = extra_deps + [ "$_dart_root/pkg:pkg_files_stamp" ]
 
     inputs = extra_inputs + [
                "$_dart_root/sdk/lib/_internal/sdk_library_metadata/lib/libraries.dart",
                "$root_gen_dir/pkg_files.stamp",
              ]
 
     output = "$root_gen_dir/$name.dart.snapshot"
     outputs = [
       output,
     ]
 
-    dot_packages = rebase_path("$_dart_root/.packages")
     abs_output = rebase_path(output)
     main_file = rebase_path(main_dart)
 
     args = [
       "--packages=$dot_packages",
       "--snapshot=$abs_output",
     ]
 
     if (dart_snapshot_kind == "script") {
       args += [
         "--snapshot-kind=script",
         main_file,
       ]
       assert(training_args != "", "Ignoring unused argument")
     } else if (dart_snapshot_kind == "app-jit") {
       args += [
                 "--snapshot-kind=app-jit",
                 main_file,
               ] + training_args
     } else {
       assert(false, "Bad dart_snapshot_kind: $dart_snapshot_kind")
     }
   }
 }