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

utils/dartanalyzer/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("../invoke_dart.gni")
 
 group("dartanalyzer") {
   deps = [
     ":generate_dartanalyzer_snapshot",
     ":generate_summary_spec",
     ":generate_summary_strong",
   ]
 }
 
 analyzer_files = exec_script("../../tools/list_dart_files.py",
-                             [rebase_path("../../pkg/analyzer")],
+                             [ rebase_path("../../pkg/analyzer") ],
                              "list lines")
 
 application_snapshot("generate_dartanalyzer_snapshot") {
   main_dart = "../../pkg/analyzer_cli/bin/analyzer.dart"
   training_args = [
     "--dart-sdk=" + rebase_path("../../sdk"),
-    rebase_path("../../tests/language/first_test.dart")
+    rebase_path("../../tests/language/first_test.dart"),
   ]
   name = "dartanalyzer"
   cli_files = exec_script("../../tools/list_dart_files.py",
-                          [rebase_path("../../pkg/analyzer_cli")],
+                          [ rebase_path("../../pkg/analyzer_cli") ],
                           "list lines")
   inputs = cli_files + analyzer_files
 }
 
 sdk_lib_files = exec_script("../../tools/list_dart_files.py",
-                            [rebase_path("../../sdk/lib")],
+                            [ rebase_path("../../sdk/lib") ],
                             "list lines")
 
 template("generate_summary") {
   assert(defined(invoker.type), "Must specify the summary type")
   type = invoker.type
-  assert((type == "spec") || (type == "strong"))
+  assert(type == "spec" || type == "strong")
   invoke_dart(target_name) {
     inputs = sdk_lib_files + analyzer_files
 
     output = "$root_gen_dir/$type.sum"
     outputs = [
-      output
+      output,
     ]
 
     dot_packages = rebase_path("../../.packages")
     build_sdk_summaries =
         rebase_path("../../pkg/analyzer/tool/summary/build_sdk_summaries.dart")
     abs_output = rebase_path(output)
 
     args = [
       "--packages=$dot_packages",
       build_sdk_summaries,
       "build-$type",
       abs_output,
       rebase_path("../../sdk"),
     ]
   }
 }
 
 generate_summary("generate_summary_spec") {
   type = "spec"
 }
 
 generate_summary("generate_summary_strong") {
   type = "strong"
 }