Format GN files and add a presubmit check that GN is properly formatted

utils/compiler/BUILD.gn

 # 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("../create_timestamp.gni")
 import("../invoke_dart.gni")
 
 create_timestamp_file("dart2js_files_stamp") {
   path = rebase_path("../../pkg/compiler/lib")
   output = "$root_gen_dir/dart2js_files.stamp"
 }
 
 create_timestamp_file("runtime_lib_files_stamp") {
   path = rebase_path("../../runtime/lib")
   output = "$target_gen_dir/runtime_lib_files.stamp"
 }
 
 create_timestamp_file("dartdoc_files_stamp") {
   path = rebase_path("../../sdk/lib/_internal/dartdoc")
   output = "$target_gen_dir/dartdoc_files.stamp"
 }
 
 invoke_dart("dart2js_create_snapshot_entries") {
   deps = [
     ":dart2js_files_stamp",
+    ":dartdoc_files_stamp",
     ":runtime_lib_files_stamp",
-    ":dartdoc_files_stamp",
   ]
 
   dot_packages = rebase_path("../../.packages")
   create_snapshot_entry = rebase_path("create_snapshot_entry.dart")
   output_dir = rebase_path(root_gen_dir)
 
   inputs = [
     "../../sdk/lib/_internal/sdk_library_metadata/lib/libraries.dart",
     create_snapshot_entry,
     "$root_gen_dir/dart2js_files.stamp",
@@ skipping 10 matching lines @@
   args = [
     "--packages=$dot_packages",
     create_snapshot_entry,
     "--output_dir=$output_dir",
     "--dart2js_main=pkg/compiler/lib/src/dart2js.dart",
   ]
 }
 
 application_snapshot("dart2js") {
   deps = [
-    ":dart2js_create_snapshot_entries"
+    ":dart2js_create_snapshot_entries",
   ]
   main_dart = "$root_gen_dir/dart2js.dart"
   training_args = [
     "--library-root=" + rebase_path("../../sdk"),
-    rebase_path("../../tests/language/first_test.dart")
+    rebase_path("../../tests/language/first_test.dart"),
   ]
 }
 
 application_snapshot("utils_wrapper") {
   deps = [
-    ":dart2js_create_snapshot_entries"
+    ":dart2js_create_snapshot_entries",
   ]
   main_dart = "$root_gen_dir/utils_wrapper.dart"
   training_args = [ "--help" ]
 }